Shortly before Christmas, Secretary of State George P. Shultz startled the executive vice president of the Heritage Foundation, Philip Truluck, with a "very cold . . . very unfriendly greeting" when the two were introduced at a holiday party. As Truluck recalled, Shultz "jumped right down my throat," inaccurately charging that the conservative think tank had called for his resignation and berating Heritage for sending him "ridiculous letters."

About the same time, Shultz surprised the nation -- and the White House -- by threatening to resign if required to take a polygraph test under an order signed by President Reagan. The following day, Reagan exempted Shultz from any lie detector tests and sharply modified his directive.

Then, a few weeks ago, Shultz erupted at presidential chief of staff Donald T. Regan at a meeting called to discuss the U.S. response to the Rome and Vienna airport attacks by a Palestinian group with links to Libya, according to reports circulating at the White House. When Regan reportedly charged that "we have no antiterrorism policy," Shultz snapped back that the chief of staff didn't know what he was talking about.

What is happening to the previously unflappable, impassive, often Sphinx-like secretary of state? Is his recent uncharacteristic behavior a sign that Shultz is preparing to leave after 3 1/2 years -- a longer tenure in the job than any of his last four predecessors, Henry A. Kissinger, Cyrus R. Vance, Edmund S. Muskie or Alexander M. Haig Jr.?

"Shultz these days often seems fed up -- tired and short-tempered. He seems more uncommunicative and less patient than in the past," said an official who sees him frequently. "There seems to be in him a little bit the sense of a man throwing caution to the winds . . . . I don't think he is looking to leave, or to stay. But I think it would take very little to trigger his departure."

Compared to his immediate predecessor -- the mercurial Haig -- Shultz has imparted an aura of calm, if not cohesion, to U.S. foreign relations. With the exception of the U.S. failure in Lebanon, the Shultz era has seen few crises and no dramatic disasters. Unlike the reign of Kissinger or Vance, the Shultz era also has seen no dramatic accomplishments for U.S. foreign policy -- no successful Arab-Israeli disengagement or peace agreements, no strategic arms treaties with the Soviet Union, no new openings to China.

The pink-cheeked, stocky Shultz, settling back wearily into a yellow wing-back chair before a crackling fire in his office at the end of a long day recently, denied that he is losing his cool and hinted at intentions of staying in office for the rest of Reagan's term.

"I would like to have the administration end with a kind of sense of continuity, that the things that have been put in place have been successful enough so that whoever succeeds the Reagan administration -- obviously I hope it will be a Republican administration -- will feel that those are the right things," Shultz said when asked about his goals for the future.

"The interests of the United States around the world are moving in a generally positive direction," he continued. "That is to say, the strength of democracy, the strength of our basic idea of freedom, the developments in the world economy, our relationships with major countries, our alliances, all have been in a positive mode."

Much of that sounds like bureaucratic boilerplate, but in the view of Shultz and many others, a rebuilding of U.S. military and economic power have brought basic improvements in the U.S. world position since 1981. The administration came to office believing that an American decline in the 1970s relative to the Soviet Union and other industrialized nations needed redressing as the groundwork for foreign policy gains.

It has been easier to obtain consensus within the administration on rebuilding American power than agreement on what the United States should do in the world from an improved position. In the absence of a chief executive with clear-cut ideas about international strategy, or who is willing to impose decisions on opposing factions within his administration, the past 3 1/2 years diplomatically have been essentially unassertive, unexciting and nondynamic.

Shultz's attributes of patience and persistence appear well-suited to such a time of relative stability. But as the Reagan administration heads into its sixth year, the international challenges of the Mikhail Gorbachev era and the internal challenges of bureaucratic deadlock and budgetary pressures may call for more imaginative efforts.

For the most part Shultz has been a manager of diplomatic relations rather than a strategist in the Kissingerian mold or an activist resembling Vance or Haig. The unpretentious Shultz's favorite metaphor for his job is that of a "gardener" of diplomacy, who persistently cultivates the soil of relations for some future bounty. Little Things 'Add Up'

Shultz's notion of foreign policy leadership emphasizes small increments and modest choices rather than dramatic initiatives. "To a certain extent what you do all day is cope," he said. "A tremendous amount of policy comes about through the way whatever little things you do all day long add up, or whether they don't add up . . . . If you have a sense of direction as you are working with the details, then there is a chance that the way the details are handled will gradually support the general line or direction you're going in."

To make the point, Shultz approvingly noted the title of an article in the Harvard Business Review: "Good Managers Don't Make Policy Decisions."

Shultz, who turned 65 in December, is the most experienced and, in some respects, the most powerful member of the Reagan Cabinet. On his chair at the White House Cabinet table are four small brass plates, each commemorating a Cabinet-level post: secretary of labor (1969-70), budget director (1970-72) and secretary of the treasury (1972-74) in the Nixon administration, and secretary of state for Reagan. Except for Defense Secretary Caspar W. Weinberger, a longstanding rival who once was Shultz's deputy in the Nixon budget bureau and who owns three brass plates on his chair, nobody else at the Cabinet table comes close.

Former undersecretary of state Lawrence S. Eagleburger, who has seen a succession of U.S. diplomats at close range, said Shultz is comparable in many respects to the two longest-serving secretaries of state since World War II, John Foster Dulles (1953-59) and Dean Rusk (1961-69). "There is a bit of the moralism of Dulles in Shultz," said Eagleburger. "And he is solid and steady and with a style reminiscent in some ways of Rusk."

To take the measure of the man and his record is difficult since so much depends on the course of policies whose foundations have barely been laid, and because Shultz's back-room methods of operation make it hard to fathom what he really is up to.

"He is one of the great bureaucratic actors of our time," said an official who has watched Shultz at close range in interagency combat. In private meetings among his peers, he said, Shultz can be blunt, even dramatic, in expressing his views. "Shultz is very self-possessed. He likes power, he likes to run things and he likes to have his way."

In inner-circle meetings, he has begun his presentation on several occasions by saying flatly, "I strongly disagree with the secretary of defense," according to a participant.

"All the stories you've read and heard about [Shultz-Weinberger] conflicts over military power are true," said an official who has watched their interaction. Moreover, on a personal level, "they don't like each other at all" despite a long history of close association in the Nixon administration and Bechtel Corp.

A Pentagon official, whose positions often clash with Shultz's, said the secretary of state is a master at inside maneuvering: "He knows when to speak up and when to hang back, and how to twist around positions to appeal to the president." Nonetheless, after 3 1/2 years of bureaucratic combat and personal interaction with the secretary, the official added, "I still can't tell whether Shultz is bright or not."

Before he chooses a position, even those closest to him have no clue as to what Shultz thinks about the questions being discussed. "I can tell when he is working a problem. He sits quiet and doesn't say anything. He just listens and is really silent," said a key aide.

An official of another agency, who used to meet regularly with Shultz, abandoned the meetings in frustration after months of one-sided discussions. The secretary of state was always courteous, attentive and patient, but never revealed his own ideas, this official recalled.

Shultz's experience and stature are major assets, but his greatest clout derives from an unusually close relationship with Reagan that developed since he replaced Haig in July 1982. Former White House deputy chief of staff Michael K. Deaver recently recalled the May 15, 1982, meeting when then-businessman Shultz, who had visited allied leaders in Europe and Japan as a special emissary to prepare for the Versailles summit meeting, reported to Reagan in the Oval Office.

"As I watched, the president just visibly relaxed with Shultz. He has a marvelous staff style that appeals to Reagan, and he is a tough guy, a good interlocutor and a consummate government official. It was clear the president was very comfortable with Shultz," Deaver recalled.

This was confirmed six weeks later when Shultz was Reagan's immediate choice for secretary of state after Haig's resignation. "If push comes to shove on important matters of foreign policy, the president usually goes with Shultz," Deaver added. Oval Office Meetings

Since August 1983, when Shultz complained bitterly that disarray in policymaking was "a disgrace," he has had a claim to at least one hour each week of the president's time for a private meeting on any subject of his choosing. This arrangement, which was inaugurated after Reagan was informed that "you have a very unhappy secretary of state," usually involves two Oval Office meetings of a half-hour or longer on Wednesdays and Fridays, to which Shultz has often invited the White House national security adviser. Such sessions are not given to other Cabinet members.

On rare occasions Shultz has used his privileged relationship with Reagan to obtain approval for diplomatic maneuvers that were unknown to competing Cabinet members, especially rival Weinberger. An example of this, according to several officials involved, was the U.S. draft communique for Reagan's Geneva meeting with Soviet leader Gorbachev. Shultz, after checking with Reagan, took the draft to Moscow two weeks before the Geneva meeting.

When Weinberger and his aides discovered that Shultz had presented the Soviets with a paper the Pentagon had never seen, they raised the roof. State reportedly was forced to disavow the paper after Reagan decided that he did not want to "trivialize" his discussions with Gorbachev by focusing on a piece of paper. It was only on the second day of the summit meeting, according to a U.S. participant at Geneva, that Reagan gave the go-ahead to draft a final communique.

Bureaucratic infighting between State and Defense is nothing new to Washington, but White House failures to referee have opened the way for a guerrilla struggle outside the normal rules of competition and left unresolved more issues than in the past. The Defense Department has become adept at delaying tactics to thwart decisions unpopular with Pentagon officials. State has retaliated by seeking to deny Pentagon officials access to meetings, cables or key decisions. Both sides have taken their arguments to the public, all of which is frustrating for Shultz, who prefers harmonious teamwork.

According to Shultz or those close to him, each of the recent incidents involving polygraphs or symptoms of irritability are attributable to different causes: his irritation with criticism from the conservative right, because he considers himself and his policies deeply conservative and because he is unaccustomed to public attacks; his deep sense of personal rectitude, which was violated by the polygraph directive; and his exasperation at being unable to bring about more forceful U.S. responses to international terrorism, a goal that has become his passion since hundreds of U.S. Marines were killed in Lebanon in October 1983.

To some degree, the "new Shultz" of recent weeks is the projection into the public arena of an assertive, argumentative "old Shultz" that previously had been seen mostly in private. For example, Shultz's celebrated anger and table-pounding at a Belgrade news conference with Yugoslav Foreign Minister Raif Dizdarevic Dec. 17 is reported by State Department sources to have been a repeat in public of Shultz's earlier private reaction to Italian Foreign Minister Giulio Andreotti. In both cases, the foreign diplomats angered Shultz by speaking sympathetically of the political causes undergirding international terrorism.

Beyond his campaign against terrorism, Shultz has most left his mark in the past 3 1/2 years on "process" -- one of his favorite words -- which in his sense means diplomatic negotiations on international problems.

A former labor mediator who believes in the importance of dialogue, Shultz has worked to open negoti- ations or to keep them going across a broad spectrum of trouble spots: the Middle East, Central America, Southern Africa and, most notably, with the Soviet Union. In many cases, these efforts drew criticism from conservative groups more interested in confrontation than dialogue with unfriendly countries.

Once discussions were under way, however, Shultz often has proven to be uncompromising in his attitudes and positions. That reflects his own conservatism as well as the political tendencies of the administration and the difficulties in obtaining a consensus.

Moreover, Shultz has repeatedly backed paramilitary actions -- for example, covert and overt U.S. aid to Nicaraguan counterrevolutionaries and covert U.S. aid to antigovernment rebels in Angola -- that tend to sharpen international conflict and complicate dialogues. At this point, none of the regional negotiations has produced agreements and some are barely alive.

Shultz seems unperturbed by this lack of success. He speaks approvingly of the Arab-Israeli disengagement agreements and Camp David accords of the 1970s as "fun" as well as "masterpieces of diplomacy." But to advance more recent negotiations would have required "concessions" on the part of the United States, Shultz said recently. "We didn't think they were quite the right things to do, so we didn't do them."

The most important diplomatic efforts generated by Shultz are the nuclear and space arms talks with the Soviets in Geneva and the broader U.S.-Soviet dialogue capped by the November summit meeting and the followup summits planned for 1986 and 1987. Like Haig before him, Shultz successfully pushed for arms and summit talks despite internal skepticism and opposition.

Since the rise to power of Gorbachev, the Soviet Union has been unusually assertive in its diplomacy and has occasionally initiated major proposals, while the United States has seemed more passive. This is a reversal of the usual roles since World War II.

A key question about the future of U.S. foreign relations is whether Shultz is capable of meeting the challenges and opportunities of this new situation. His record and historical reputation as secretary of state are likely to depend in part on whether the Sphinx can rise to this occasion with energy and imagination.

NEXT: Terrorism and polygraphs.