Thanks to the FBI's undercover "sting" operation, there now exists incontrovertible evidence that one senator would not be bought.

Preserved among the videotape footage that may be used as bribery evidence against a number of members of Congress, there is a special moment in which Sen. Larry Pressler (R-S.D.) tells the undercover agents, in effect, to take their sting and stick it.

Pressler, according to law enforcement sources, was the one approached member of Congress who flatly refused to consider financial favors in exchange for legislative favors, as suggested by undercover agents posing as Arabs. At the time, he said, he was not aware that he was doing anything quite so heroic.

"I'm just confused in my own mind," Pressler said in an interview yesterday. "At first I thought because they were Arabs that they were interested in gasohol -- which I am very big on. . . ."

The story of how a U.S. senator came to be in the lair of the FBI's "sting" house involves a mid-afternoon dash from the Capitol to fashionable Foxhall, a middle-aged Georgetown woman who is described as a name-dropper and who became a go-between, and a companion who Pressler know only as "the big, burly man"

As Pressler recalls, it was around the 7th or 8th of November that he got a message relayed by one of the fund-raisers for his since-folded presidential campaign that some people wanted to see him to discuss campaign contributions.

He said the message came through a roundabout channel: It was telephoned to his office by his fundraiser, Harriet Dent; she had heard it from her ex-husband, Edward (Chip) Dent III, and he had heard it from the woman who lives across the street from Dent's home on N street in Georgetown. It was this woman, Marilyn Bell, wife of Dr. J. Gordon Bell, a surgeon, who was to become the intermediary between Pressler and the FBI's undercover house.

"She [Bell] called me . . . and asked me if I know Larry Pressler," Den recalled."She said, 'I have some friends who are interested in meeting various candidates and making contributions.'. . . She started throwing around some heavy numbers: $5,000, $10,000, $15,000, $29,000. . . . She said they want to meet the candidates, no strings attached, and want to make contributions."

Dent said he had known the Bells as neighbors and had no reason to doubt what she was saying.

"I know she used to drop names a lot," Dent said. "She gets a kick out of . . . getting involved with politicians." Now that the sting operations has been made public, Dent said, he is not sure what to believe about his neighbor.

"Either she was working for the FBI or she's been a dupe," he said. Mrs. Bell was not at home yesterday and could not be reached for comment.

Pressler said that after receiving the mssage, he agreed to meet Mrs. Bell and a friend of hers at 2 o'clcok that afternoon in the reception room outside the Senate chamber. Pressler said he was delayed until about 2:30 p.m., by which time Mrs. Bell and her friend were no longer there.

The senator says he had his staff contact Mrs. Bell by telephone and a second meeting was set up -- for 4 p.m. that day on the east steps of the Capitol. This time, Pressler said, he met Mrs. ybell. He said she told him they had to go and meet her friend, who was nearby and then they would go to a house near the Capitol to meet the potential contributors.

The senator said he climbed into her Mercedes and she drove him toward Georgetown. Pressler said this surprised him, but that he agreed to continue with the rendezvous.

They stopped on M Street in front of Clyde's, a restaurant and bar, where a man got into the car. Pressler remembers him only as a "big, burly man."

Pressler recalls: "He said he wanted me to talk with some people who represent Arab interests . . . that they have given to Democrats before and about that. . . .' I said that would be illegal. I kept mentioning the word 'illegal.'"

Pressler said that the two "Arabs" never specifically offered money in exchange for legislative favors, but instead kept making "open statements." He said: "They didn't exactly offer me anything. But they did say we have American citizens who are Arabs and who could raise money for your campaign -- they could raise a lot of money. And I said, 'That would be illegal' and they said, 'Don't you know how it could be done?'"

Pressler said that at one point the men said they could easily raise $50,000.

Pressler said repeatedly that the two seemed "very flaky." He said, for example that they did not seem to know who he was -- whether he was a senator or a congressman.

Said Pressler: "Finally, I said I have to go, and we got back in the car. Inside the car, Mrs. Bell said, 'How did it go?' And the burly guy said, 'Well Pressler blew that one.'"

Later, Pressler said, "The thought crossed my mind of reporting it [to law enforcement authorities] but no offer was actually made."

He added: "The whole thing is very scary. I mean . . . if you tried to even be polite to them, you might say something . . . The FBI hasn't even contacted me . . . I'm a little upset because I'm very clean. My campaign records are kept perfectly. Why would they want to test me out? There's nothing in my background.

"I'm very upset at the FBI."

A law enforcement source, while declining to discuss the details of Pressler's account, said that Pressler was not a subject of the FBI investigation and added that Pressler is "an honorable man."