SAN JOSE, COSTA RICA -- In December 1986, when Costa Rican President Oscar Arias Sanchez was visiting Washington, the late CIA Director William Casey summoned him over to Langley for a secret one-on-one meeting.

Arias refused to go.

Instead the Costa Rican, winner this month of the 1987 Nobel Peace Prize, said he would see Casey in his Westin Hotel presidential suite. There the aging spy master was greeted by a noisy roomful of Costa Rican officials, Arias' entire delegation, all dying to ogle the renowned chief of American intelligence.

For half an hour Casey perched awkwardly next to Arias in front of the delighted spectators as the two men exchanged what one Costa Rican remembered as "laughable pleasantries." Casey apparently expected Arias to dismiss the crowd so they could get down to business. But Arias never did. Casey, smoldering, finally left.

Little wonder Washington conservatives regard Arias, main author of a peace accord signed Aug. 7 in Guatemala by Central America's five presidents, as a closet communist and insufferable upstart. With his peace plan and laurels, Arias has put the Reagan administration's passionate policy of support for the war waged by the Nicaraguan rebels, or contras, into checkmate.

But Arias' aides argue the 46-year-old president of Latin America's oldest democracy wasn't being cocky with Casey that day; he was just exposing the late CIA director to a few basic elements of his political thought. For one thing, Arias has no taste for covert cabals, an unexpected attribute in the head of a country best known up to now as the former hideout of fugitive financier Robert Vesco and one-time rear guard of contra comandante Eden Pastora.

Arias also believes, in defiance of common Central American wisdom, that he and his nation of 2.6 million have enough stature to enter into a gentlemen's disagreement with the White House -- and get away with it.

The Dec. 5 meeting was not the first time Arias gave Casey a hard time. On Oct. 6 of the same year, Casey roared unannounced into the San Jose airport aboard a U.S. Air Force Galaxy, just about the biggest aircraft ever to land in Costa Rica -- and demanded a top-secret meeting with the president. That time Arias left Casey high and dry, sending his foreign minister out to field the CIA director's petitions for more sub rosa Costa Rican collaboration with the contras.

"We were trying to rebuild Costa Rica's credibility as a neutral country, to establish some authority to become a regional mediator. We wanted to show we are not just a blind instrument of the Yankees," said a senior Costa Rican diplomat.

Last June, before the peace plan was signed, President Reagan called Arias to the White House when the Costa Rican was passing through the United States. Reagan expected to remind Arias that he was determined to back the contras. Instead, in the presence of Reagan's top foreign policy team, he got a half-hour oration from Arias on why the contras would never be able to vanquish Nicaragua's leftist Sandinista government.

Recalled John Biehl, Arias' Chilean speechwriter and alter ego who also attended the White House meeting: "Oscar was talking, and Reagan kept looking over at his advisers as if to ask, 'Who let this midget in here?' "

Long before he became a prophet of peace, Arias was an egghead with a determination to lead his country but none of the appeal he needed to do so, according to both detractors and friends.

"I always said I would be president, but many government ministers made fun of my pretensions," Arias said in an interview at his home.

His younger brother Rodrigo, now his chief of staff, remembered that Arias first unveiled his plan to be head of state in his 1958 high school yearbook. Years later, in January 1986, at a time when Arias was lagging behind in his presidential race at home, he attended Vinicio Cerezo's inauguration as president of Guatemala in that country's capital. During the ceremony Arias wanted to meet another guest, Vice President George Bush. Arias says he plunged through a wall of Secret Service men and grabbed Bush's hand, introducing himself as "the man who will be the next president of Costa Rica."

His drives were nurtured at his family's hearth in the aristocratic San Jose suburb of Heredia, where he still goes most Sundays to lunch with his parents. Like many Latin men, the president still calls his mother "Mammi." One of Arias' grandfathers was a self-made coffee baron and the family is still monied. The house is somber, all mahogany sideboards and gray stuffed chairs. Framed on one wall are different currency bills issued and signed by three generations of Ariases: a grandfather, Oscar's father and later Oscar himself all held the country's top treasury post.

A ponderous pewter rendering of the Last Supper on a maroon velvet background looms over the dining room table, a reminder to all present that Catholic morality doesn't let up, not even during meals.

"When Oscar Arias first came on the political scene in Costa Rica, he was a real novelty: a colorless, introverted man with no known leadership qualities and no roots in his own political party," said Rolando Lacle, a lawyer who helped manage the losing campaign of Rafael Angel Calderon, a conservative Christian Democrat who ran against Arias for president in 1986.

Arias said that once in the mid-'70s, when he was a young minister under Costa Rican patriarch and then-president Jose Figueres, the two men went to a bridge christening. Arias, with a fresh graduate degree from the London School of Economics, gave a high-toned speech to a pool of impassive peasant faces. Then the practiced populist Figueres got up and announced he would translate what Arias had said into Spanish everyone could understand, drawing hoots of joy from the grass-roots gathering and a hot blush from Arias.

"It was hard for me to make people feel I was just one of them," Arias said. He claims: "It wasn't arrogance, it was timidity."

But Arias wasn't timid about maneuvering his way in record time to the top of his social democratic National Liberation Party. He won the 1985 nomination for presidential candidate by committing the impudent heresy of crossing party elders, among them his mentor Figueres, who wanted to run again despite his advanced age.

In fact, modesty hasn't been one of his strong suits. In the days after his peace plan was signed, he was asked in an interview how accord was achieved among the notoriously disputatious presidents in the Aug. 7 meeting. Arias gave a one-sentence reply: "They signed because I gave them a really great proposal."

At his home on a recent Sunday, Arias was eager to show some television spots he used during his presidential campaign to demonstrate that peace was already one of his central themes back then. But as soon as the ads began to play on his video recorder screen, he became utterly mesmerized by the publicity images of Arias with cheerful children hanging on his neck, Arias before the microphones, Arias acclaimed by a crowd of 300,000 party sympathizers. He sat in reverent silence for 15 minutes as he watched all eight ads.

"Can Ronald Reagan tell you off the top of his head what year Karl Marx was born?" Arias asked rhetorically over lunch. (It was 1818.) He showed off his library, quoting from memory such favorite authors as Karl Popper and Isaiah Berlin; he quoted them from memory. "Tell them," he said, "that the president of Costa Rica is a man who could be president of any European country."

He can't resist the Latin man's temptation of presenting himself as both model family man and reckless rake. By all accounts his wife Margarita, who took a BA in chemistry from Vassar, is a thoughtful member of his political inner circle who taught him, among other things, some of her easygoing warmth with the public. The president, a short-statured asthmatic with a jowly frown and melancholy tone of voice, hardly fits the role of irrepressible macho. Nevertheless San Jose is rife with gossip of presidential peccadilloes, rumors happily fueled by Arias' top aides. They like to point out with a wink that Costa Rican women were one of his most important voting blocs.

Still, behind such ego props lies a substantial intelligence that Arias has trained on enhancing the standing of his country. Under his predecessor, Luis Alberto Monge, Costa Rica was just one more isthmian pocket nation that pacted with the American devil to help the contras in exchange for U.S. economic aid.

Arias, by contrast, cuts his course closer to a more cerebral Costa Rican tradition of peaceful neutrality, the same wellspring that inspired abolition of the national army in 1948. "Peace builds," read the plaques on his new public constructions, reminding Costa Ricans his peace plan is also partly a strategy to stimulate economic investment.

Moreover, Arias seems uninterested in the perquisites of power. A Sunday afternoon found the head of state in a leather jacket and loafers, driving himself across San Jose to his parents' home in a plain Jeep station wagon, windows rolled down. No chauffeur, no sirens, no zooming machine-gun-armed motorcycle convoys like those that accompany the other violence-conscious Central American presidents. In fact, no Costa Ricans on the tranquil streets that day seemed to notice their president at all.

Late that Sunday he drove back to his residence at dusk just as his presidential contingent, totaling four guards, had stopped traffic momentarily in the street outside for a routine two-minute flag-lowering ceremony. Arias fumed at the delay. He muttered: "Those people are sitting waiting in their cars and wondering: Who does the jerk who lives in that house think he is?"

The first draft of the peace plan that won Arias the Nobel Prize was composed on a napkin in the cafeteria of Washington's Mayflower Hotel in September 1985, eight months before Arias was elected president. Arias and two aides were on their way to a panel discussion, and decided right then and there to hammer out a position on the regional convulsion centering on Nicaragua's Sandinista government.

They agreed in the scribbled notes, according to Housing Minister Fernando Zumbado, who was there, that the contras would never succeed in driving out the Sandinistas. So Costa Rica would have a chronic war on its northern border if it did not aggressively push a diplomatic settlement.

There were many more drafts -- reportedly adviser Biehl wrote the one that Arias polished and made his final plan. Another core belief behind it is that Costa Rica, with only an 8,500-member police force, could never excel in the arts of war but might make its mark at peace. Aides say Arias felt the U.S. contra policy was an effort by an ambivalent post-Vietnam Washington to use Costa Rica as a proxy to defend American security interests. But if Washington truly wanted guarantees against the Sandinistas, Arias reasoned, the administration should deal directly with Nicaragua.

From those early beginnings Arias and his Young Turks sensed their plan would put them on "a collision course," as Arias puts it, with Reagan, the contras' most unflagging supporter. But he wasn't dissuaded.

"Oscar always has said the United States is history's most benign empire. It tolerates dissent," Biehl explained.

On one hand, Arias knew there would never be any deep tensions with Washington because Costa Ricans, perhaps more than any other Latin nation, adore the United States. One wall slogan in San Jose reads, "Yankee go home," and then after it, in Spanish, "And take me with you!"

Also, events handed Arias some leverage over administration officials. Before he took office in May 1986, Arias was told by the outgoing Monge and then-U.S. ambassador Lewis Tambs that an airstrip was being readied in northern Costa Rica for resupply missions for the contras. The United States also hoped to build an antenna in Costa Rica to intercept Sandinista radio messages and pass them on to the contras, and third, it wanted Costa Rica to give preferential treatment to contra war wounded in its social security hospitals. Arias nixed all three projects.

During that period, Costa Rican officials say, Tambs invited Arias to declare him persona non grata if the airfield was ever used without Arias' permission. Assistant Secretary of State Elliott Abrams once came to Costa Rica and said the U.S. government was not involved in the contra airdrops. Then the press and the Iran-contra hearings revealed that the airstrip was used without Arias' permission and the Reagan administration was involved. So, Arias' aides said, the president eventually knew that Tambs, who resigned, and Abrams had lied.

Arias confesses he is not awed by the threat posed by Sandinista Nicaragua. As a scholar he pondered and rejected many Marxist texts that compel the Sandinistas. Nor are they strangers to him: Nicaraguan Vice President Sergio Ramirez published two of Arias' books when Ramirez was an exiled editor in Costa Rica in the 1970s.

Arias thinks the East-West clash in the region can be managed. Critics say the most dangerous loophole in his peace plan is its lack of restraints on Soviet bloc military aid to Nicaragua. But Arias says he left that out for a practical, not ideological, reason: He would also have been obliged to include a clause forcing other regional governments to give up their military aid from the United States.

El Salvador's President Jose Napoleon Duarte, a close U.S. ally who recently kissed the American flag in Washington, "would have thought I was trying to get rid of him," Arias chuckled.

Perhaps Arias' most singular contribution was to discover the unlikely brotherhood of the Central American presidents. Arias and his Nicaraguan counterpart Daniel Ortega have both said openly and often that they despise each other's politics. But as two young buck Central Americans seeking margins of independence from the United States, they have found enough common ground since Aug. 7 to parlay. Ortega, who is almost the same age as Arias, was the first head of state to congratulate him on the Nobel Prize, reaching the still astonished laureate by phone at his beach house the morning of the Oct. 13 announcement.

Arias said he seized the moment to ask Ortega to open mediated talks with the contras, which the Sandinistas so far have categorically refused to do. Arias says Ortega agreed to try to do so, but cautioned he needed time to prepare his followers at home.

Costa Rican officials say so far they have paid no price in U.S. aid or cooperation for their insubordination, while gaining the unparalleled prestige of the Nobel. However, they say their diplomats in Washington have been virtually frozen out by the State Department since August.

"President Reagan and Secretary {George} Shultz sent me kind messages of congratulations about the prize. But I wonder what is in their hearts. I suspect un poco de mixed feelings," Arias said.

Now Arias has a prize for a peace that hasn't happened yet. He doesn't know how to shoot a gun, and as president he has never had to order any Costa Rican to kill another in any military operation. With the peace plan's Nov. 7 deadline less than two weeks away, he has only his brains and tenacity to face his greatest test.