Last Sept. 25 was a terrible day for Georgi Mamedov, a poised, articulate 36-year-old Soviet diplomat described by American counterintelligence officials as an operative of the KGB in Washington. That was the day Georgi Mamedov's wife, Irina, took their 5-year-old daughter, Tatiana, and defected to the FBI.

FBI agents quickly whisked Mrs. Mamedova and her daughter to a heavily protected safe haven more than 25 miles from Washington, beyond the area in which Soviet officials are permitted to travel freely. Several days later, Soviet security agents put Mamedov onto a plane for the Soviet Union, where he is likely to remain for many years.

The FBI hoped it was about to score a coup in the intensifying battle between Soviet and American intelligence agencies--the first defection by the wife of a KGB agent stationed in Washington. But the leading lady in this drama rewrote her part.

Soon after her defection, Mrs. Mamedova--who spoke English well, and called herself "Irene" in dealings with Americans--talked to American friends by telephone. She had previously spoken openly with them about her marital difficulties. She told the friends that she defected first of all for her daughter. She picked Sept. 25 to flee to the FBI, Mrs. Mamedova said, because she and her husband were scheduled to return to Moscow for an extended leave in the next day or two.

She also said she had no intention of ever meeting with officials from the Soviet embassy to explain her action. Such meetings are common, but the U.S. State Department allows a defector to decide whether or not to hold one.

The next 12 days were traumatic ones for Mrs. Mamedova, a dark-haired 35-year-old whom American counterintelligence sources described as neither well educated nor particularly strong in character. She had fled to the Americans not for any political reason, but because she wanted to get away from a husband who regularly got drunk and beat her, according to American officials.

Somehow, Mrs. Mamedova's original certainty about her defection dissolved during the subsequent 12 days. She agreed to have a meeting with Soviet officials, and the meeting took place on Oct. 7. According to American officials, the Soviets at this meeting told Mrs. Mamedova that they knew about the difficulties in her marriage, appreciated the emotional strain she must have been under, and promised that if she returned to the Soviet Union, no reprisals would be taken against her or her family.

At the end of that meeting she decided to return to her homeland. She left the State Department in the custody of the two Soviet officials.

One senior American counter-intelligence official described Mrs. Mamedova as "a classic case of the battered wife . . . We weren't exactly sure what we had," the official added. These American officials said Mrs. Mamedova could not cope with the trauma of being completely cut off from her past life and personal associations, and decided to go back to Moscow.

Friends who talked with her soon after her defection had no further contact before her decision to go back home, and could offer no explanation for her abrupt change of heart.

A former Soviet defector who fled to the West several years ago said in an interview that those first days after a defection are filled with emotional strain, and that a new defector--particularly a wife and mother with no strong political feelings and very few ties to American life--would have to be handled with great delicacy by American counterintelligence officials. This earlier defector speculated that the pressures just may have been too great, particularly if Mrs. Mamedova felt in any way misunderstood by her new protectors.

A senior U.S. official said Mrs. Mamedova and her daughter would have received American asylum if she had requested it, but more for humanitarian reasons than for the potential intelligence information they might have gained from her.

Other former Soviet citizens, including the earlier defector, said she has probably been harshly punished for committing an act the Soviet Union regards as high treason. American counterintelligence sources said they had information indicating that Mrs. Mamedova would probably lose custody of her daughter.

Georgi Mamedov's promising career may have been destroyed, American officials and former Soviet citizens said. As American counterintelligence sources put it, KGB men stationed in Washington are not permitted personal transgressions like drinking too much or mistreating a wife.

However, Mamedov enjoys special status in Moscow as the son of an important government official, so he may yet be able to salvage his career, some sources said.

Mamedov is the son of Enver Mamedov, who for 20 years has been first deputy chairman of the State Committee on Radio and Television, the body that runs all radio and television programming in the Soviet Union. This background means Mamedov has been raised in the lap of privilege and comfort reserved for the Soviet upper crust. He grew up in a large apartment on Komsomolsky Prospect in Moscow and attended the elite Institute for International Relations attatched to the Soviet Foreign Ministry.

According to a former colleague of Mamedov's at the Institute for the U.S.A. and Canada in Moscow, his father's connections helped him get assigned to the Soviet embassy in Washington in 1977.

In Washington, Mamedov had the low diplomatic rank of third secretary, but unlike other junior Soviet diplomats, he was assertively self-confident and outgoing. He told Americans to call him "George," and said he was a junior representative of the Institute for the U.S.A. and Canada in Washington. Later he was promoted to second secretary and described himself as an assistant press attache. FBI sources in Washington said he actually worked for the political information section of the large KGB station in Washington.

The KGB is the Soviet secret police. Its initials in Russian stand for Committee for State Security. But it is a much bigger and broader organization than the American FBI or CIA, and includes many operatives who are not cloak-and-dagger spies. KGB agents like Mamedov whose job it is to cultivate Americans tend to operate more like conventional western diplomats, whereas ordinary Soviet diplomats are usually more cautious about their Washington contacts.

Mrs. Mamedova also had well-connected parents in Moscow, according to Galina Orionova, 35, a former colleague of Mamedov's in Moscow who knew the couple there. Galina Orionova defected to the West in London in 1979, and is now studying for a Ph.D. at Oxford University in England.

According to Orionova, the Mamdedovs were married in 1973 or 1974 (it was her second marriage), and "it was not a very happy marriage from the beginning . . . . They had a lot of personal tensions. She was very pretty, very sexy, but not very intelligent or clever, sort of a pretty doll--a bit nervous, a bit pretentious, flirting a lot. There were rumors that she was nasty to him."

An American who got to know the Mamedovs quite well described Irina as "a playgirl" who loved to dance, and made no secret of her appreciation for the material pleasures of western life. She gossiped freely about her marital problems and liked to flirt with American men.

Mamedov, his fellow-student Orionova said, was a bright and successful student in Moscow who wrote his "candidate's" dissertation--for a degree roughly equivalent to a master's--on Soviet-American summitry. "He was very cynical, like a lot of people at the institute," Orionova said. He was active in the young Communist League branch at the institute, and "it was clear that he was waiting for a place in the sun," she added.

Often, she added, the children of Soviet bigshots are unpopular with their fellow-students because they assume superior airs. But Mamedov was well liked, she said.

In the first years of his assignment in Washington, Mamedov concentrated on the strategic arms limitation treaty. He regularly attended Senate hearings on SALT II during the summer of 1979, cultivated Senate aides and arms control specialists in Washington, and made a number of speeches around the country supporting the treaty. His good English and ready command of the jargon of the SALT debate made him a most unusual Soviet diplomat. He aggressively pursued his contacts here both to lobby for SALT II and to gather information on the course of the Senate's deliberations on the treaty.

He sought contacts among opponents of the treaty as well as its supporters. He once dined at the home of Richard Perle, then an influential aide to Sen. Henry M. Jackson (D-Wash.). Jackson was a leading hard-line opponent of the treaty, and Perle was a principal strategist for anti-SALT forces in the Senate.

Perle, now assistant secretary of defense for international security policy in the Reagan administration, said Mrs. Mamedova joined her husband for the dinner at Perle's house. "She was quiet, and seemed a nice person," Perle recalled.

Mamedov made strikingly different impressions on the Americans he knew in Washington. Alton Frye, who heads the Washington office of the Council on Foreign Relations, called him "one of the smartest of the Soviet figures we've had here, and one of the most judicious . . . an informed and well-meaning man."

A senior diplomatic correspondent here who asked not to be named said Mamedov's pretensions to knowledge of the American political system far exceeded his true grasp of what was going on here. "Only after Afghanistan," said this journalist, "was it possible to have a good conversation with him." The dramatic American response to the Soviet invasion of Afghanistan showed Mamedov that there was a lot about the United States he still didn't understand, this American said.

With aides to key senators on the Hill, Mamedov was "all business," as two of them put it. Some aides found him well-informed and "very westernized," in the words of one of them. But another aide steeped in the lore of arms control and SALT said Mamedov's knowledge was superficial, and that he often insisted on talking "in declaratory terms, saying things like 'we must have arms control, we must have detente.' "

The FBI contacted some of the people Mamedov knew in Washington. Late last summer the FBI was interested in knowing what Mamedov's future career might be.

At one point he told a Senate aide that he was in line to become the Soviet ambassador to Spain. Later he told the same man he had received a more attractive offer of an important job in Moscow. A scholar still attached to the Institute for the U.S.A. and Canada has told western colleagues earlier this month that Mamedov is now attached to the Foreign Ministry in Moscow.