The Prentices, a couple from Britain on their first trip to Washington, itched to see the Smithsonian, the FBI and Congress. What really caught their fancy, though, were the tapes of Richard Nixon.

"We followed Watergate quite closely at the time," Gordon Prentice, 30, said the other day, after hearing John Dean alert Nixon to a "cancer on the presidency," a high spot in the White House conversations. "But this," he added with a Scottish lilt, "this is completely fascinating."

Much to the surprise of the folks at the National Archives, who had planned to end the tapes' 14-month run at summer's end, people still are flocking to the weekday sessions --and still giving the 31 tapes rave reviews.

"We expected interest to drop off, but it really hasn't," said Archives spokesman Ben Ruhe of the recordings, whose 121/2 hours are played over a seven-day cycle. "So instead of retiring the tapes and making them available on individual request, we'll probably be playing them indefinitely."

Every week, between 75 and 100 people show up at the building's Pennsylvania Avenue entrance to hear the tapes. In all, about 15,000-- from as far away as Australia and Nepal -- have done so since theArchives started playing them in May of last year.

Serving as host for the sessions, which are held in a second-floor reading room, is one of the 25 professional archivists involved in cataloguing the 6,000 hours of tapes and 20,000 cubic feet of presidential papers yet to be made public. On duty recently was Scott Parhum, 50, a stocky ex-history teacher with a shock of red hair.

After a chatty introduction --"Where're you from?" "Been enjoying your stay?" -- he explained that 30 of the tapes span a period from June 23, 1972 -- six days after the Watergate break-in -- to April 19, 1973, that they were made public by an act of Congress, and that all 30 were used as prosecution evidence in the trial of several Nixon associates (The remaining tape -- a discussion among Nixon, John Connally and other officials in the Oval Office on March 23, 1971 --concerns price supports for milk producers).

"The case is called 'The United States vs. John Mitchell et al', which is just legal language for 'andothers' , " Parhum said, taking on the tone of the pedagogue. "Mitchell was Attorney General. Anybody know who the 'and others' are? How about you?" He pointed across the room.

"Well," said the surprised visitor, squirming. "There was H. R. Haldeman, uh, John Ehrlichman, and, I think, Mardian."

"Mardian?" Parhum said, repeating the name of Mitchell's trusted deputy, Robert C. Mardian. "Are you a lawyer? Only the lawyers remember Mardian."

Jim Hastings, deputy director of the Nixon Project, said that, aside from tourists, the tapes have attracted a number of other inquisitive folk in recent months -- summer congressional interns, scholars from Finland, and a contingent of African journalists who emerged from their session "with glazed eyes," he reported.

Most react favorably, but last November, Hastings recalled, a curmudgeonly gentleman interrupted a session to declare: "You're just playing these tapes to help the Democrats win the election, and what you're really doing is bringing a bunch of degenerates off the streets to fill the room."

That got chuckles from a phalanx of Justice Department lawyers who happened to be sitting up front, Hastings said.

Two weeks ago, Samuel Dash, the former counsel to the Senate Watergate Committee and now a professor at Georgetown University, dropped by to hear them for the first time-- eight years after his staff coaxed the secret of the tapes' existence from White House aide Alexander Butterfield.

"I didn't want to go over there just by myself," said Dash. "You know, since I had such a major role in the thing, I just wouldn't have felt comfortable about it. But when one of the Georgetown faculty members told me a group of them was going, I thought, 'Well, maybe I just ought to go over, too.' "

After attending three sessions, he said, he is glad that he did.

"There's quite a difference between reading the cold print in the transcripts and actually hearing the voices and intonation -- the conspiratorial tone of the voices," he said.

"Shortly afterward, I sat down and wrote a letter to Sen. Sam Ervin," he said, referring to the North Carolina Democrat who chaired the Senate Watergate Committee. "I said to him, 'They should be heard by all Americans every once in a while, just to remind themselves what happened in this country, so we can get a new pledge of vigilance.' "

Not all of those who were intimately involved in Watergate are so eager to listen to the tapes, however. Former Nixon aide Charles Colson, for instance, whose voice shows up in the tapes in a telephone conversation with E. Howard Hunt in November 1972, said through a spokesman last week, "I haven't been there to listen to the tapes, and I don't plan to."

There are many ordinary citizens, though, who think the experience is well worthwhile.

"This is better than a soap opera," said Karen Wiebler, a 29-year-old learning disabilities specialist from Fairfax County, who was determinied to attend every session.

"It makes for an enjoyable morning," said Jim Goodman, 30, a police officer from New Berlin, Wis.

As for the Prentices -- Gordon, a government employe in London, and Bridget, a high school history teacher -- they already had visited twice and were thinking of coming again.

"It's amazing to us how inarticulate and disjointed Nixon sounds," Gordon said. Bridget added, "When we go back home, we'll certainly recommend it to our friends."

Hastings said that, while the tapes "no longer have a great informational value," they still are a touchy subject at the Archives. Three lawsuits concerning the Nixon presidential collection there -- two brought by Nixon and one by former White House chief of staff Haldeman --currently are in various stages of litigation. Both men seek financial compensation from the General Services Administration, while Nixon also is attempting to limit public access to the materials.

Hastings said that the 31 private conversations, which Congress released to the public over Nixon's protests, still could have a bearing on the suits.

"I want to make sure that nobody says something wrong," he said in explaining why he declined to let his staffers talk to a reporter. "We just say what the tapes are. We don't interpret them."

Not everyone who listens to the tapes would agree that they are worthy of such attention, however. To 11-year-old Diane Vanderlugt, of Grand Rapids, Mich., they are a huge yawn.

Diane, who was four years old when Nixon resigned, came the other day with her family to hear the "cancer on the presidency" talk of March 21, 1973, put on her headset and promptly fell asleep.

"I guess it was all right," she said upon waking up.