Every 20 minutes of the business day a chartered bus shuttles men in uniform and pin-stripe suits to the high temples of the military-industrial complex.

From the Pentagon, the bus sweeps south one mile to the Crystal City office suites of scores of defense contractors and consultants, then returns to the Pentagon, delineating what President Dwight D. Eisenhower described 25 years ago today as the "conjunction of an immense military establishment and a large arms industry."

Eisenhower, in his farewell address as president Jan. 17, 1961, noted the emergence of a "permanent armaments industry of vast proportions" in the United States and acknowledged the "imperative need" of industry and government to manage it jointly.

Today, the military-industrial complex -- in Eisenhower's enduring phrase -- is larger and more pervasive than he could have imagined. Linked by profit and patriotism, the armed services, corporations, scientists, engineers, consultants and members of Congress form a loose con- federation that reaches almost every corner of American socie- ty.

A few measures cited by defense analysts illustrate the power of the complex. The Defense Department is the largest single purchaser of goods and services in the nation. One of every 20 jobs is directly or indirectly tied to defense spending. Defense employs as much as a quarter of America's scientists and engineers. In several states, defense employment is one of the largest sources of personal income.

The Pentagon plans to spend nearly $300 billion this year, including $34 billion for research and development -- a sixfold increase over Eisenhower's last budget in constant dollars -- spending that, critics charge, deepens the national debt and diverts resources from needy social programs.

Eisenhower also spoke of the "grave implications" of this union, and in a warning that has become a benchmark for defense analysts and social scientists alike, said:

"In the councils of government, we must guard against the acquisition of unwarranted influence, whether sought or unsought, by the military-industrial complex. The potential for the disastrous rise of misplaced power exists and will persist."

But despite its size, a statistical inventory of the military-industrial complex can be misleading in assessing its influence. For, despite its awesome dimension and extraordinary growth, the defense community today accounts for a smaller share of federal spending, national research and development and labor force than it did in 1961.

Whether this confederation has gained "unwarranted influence," as critics charge, by skewing national priorities, or whether it combines the forces needed to undergird national security, is an issue of intense debate a quarter-century after Eisenhower's admonition. That debate has been one of Eisenhower's most enduring legacies.

"It's defending our freedom," said Deputy Defense Secretary William H. Taft IV. "It's what brings us peace, it's what keeps the Soviets interested in arms reduction and protects our interests. What could be more valuable?"

Jerome B. Wiesner, former president of the Massachusetts Institute of Technology who served as science adviser to Eisenhower and President John F. Kennedy, contends that pressures from the military, industry and Congress often drive national decisions to build unneeded weapons systems that undermine arms control.

"It's no longer a question of controlling a military-industrial complex," Wiesner wrote recently, "but rather of keeping the United States from becoming a totally military culture."

The term "military-industrial complex" in popular usage has come to describe the merger of three main partners with a vested interest in the production of weapons: the armed services that use them; the defense industry that profits from building them, and members of Congress who gain constituent favor by landing military installations and defense factories for their districts.

Together they form what defense analyst Gordon Adams calls the "iron triangle," a powerful alliance that, he says, perpetuates itself and grows by advancing the common interest.

But the complex is hardly monolithic. The Joint Chiefs of Staff, shedding their hawkish reputation, have been a voice of restraint in the Reagan administration on matters ranging from continued observance of the SALT II arms control treaty to military reprisals against Third World terrorists.

"It has become more complicated since President Eisenhower issued his warning," said Paul C. Warnke, director of the Arms Control and Disarmament Agency in the Carter administration. "If you look at it today, the most rational people are in uniform. They recognize as military commanders that there's no sensible way to use nuclear weapons." In the procurement of those weapons, however, the military and its civilian managers readily join forces with industry and key lawmakers. It is in the growth and management of the nation's arsenal, critics said, where the "iron triangle" has gained undue influence.

Among defenders of the system, retired admiral Thomas H. Moorer, chairman of the Joint Chiefs in the early 1970s, said that armament decisions are driven by perceptions of the "threat," not by bureaucratic or corporate pressures.

"You don't work the problem backwards by building weapons and then formulating a policy to use those weapons," he said.

According to Wiesner, however, the Pentagon has exaggerated the Soviet threat to justify unnecessary weapons systems ranging from Eisenhower's B70 bomber to President Reagan's MX missiles. Prodded by self-interested military and industry leaders, he said, the "United States has been running an arms race with itself."

Other critics argued that the unhealthy collaboration of Pentagon and industry continues throughout the life of a weapons system, often resulting in bloated costs and substandard work.

Congress, while sharpening its scrutiny of defense spending in recent years, is generally eager to approve new weapons programs that mean jobs and income for constituents. The Navy assured support for its costly fleet expansion after dispersing new ports to the districts of lawmakers on key military committees.

"Some perfectly useless weapon systems continue to be funded because a member stands up in a closed meeting and says, 'Look, guys, I have to have this for my district,' " said Rep. Les AuCoin (D-Ore.), a member of the House Appropriations defense subcommittee.

Karl G. Harr Jr., president of the Aerospace Industries Association who served as an Eisenhower aide, said a close partnership of government and industry is natural if the nation is to remain secure.

"I would recognize there would be a bias from those who feel defense is important," he said. "That's the way democracy works.