When a congressman retires or is voted out of office, it can cost the taxpayers a lot of postage to ship him home for good.

That's because it has become a common practice among such lame-duck congressmen to take some of their remaining expense account in the form of postage stamps.

For example, in the weeks before their public careers ended at noon last Jan. 3, eight House members with a total of 155 years of experience in Congress used their office expense accounts to obtain $53,405 worth of stamps from the House postmaster.

For that amount of money, a postal clerk will give you 356,333 15-cent stamps -- and a nickel change.

House staffers are split over whether the practice is a violation of fundamental House rules or merely, as one aide called it, "highly unethical."

Normally, members of Congress have no official need for so many postage stamps. They enjoy a franking privilege that allows them to send official mail without stamps at half the normal cost to the Postal Service.

And to allow the departing members to clean up all remaining official correspondence, the House rules permit them to use the frank for 90 days after they leave office.

Still, there was a huge demand for stamps at the House postmaster's office from congressmen as they were about to leave office. All of them signed vouchers saying that the stamps would be used for an "official and representational duty."

Former representative Robert N. C. Nix for instance, knows something about postage stamps. The Philadelphia Democrat the House Post Office Committee.

Nix, a congressman for 20 years before being defeated in a primary last year, obtained $11,100 worth of stamps at public expense during the four months before he left office.

Asked to explain, Nix told a reporter he hadn't "the vaguest notion" what happened to the stamps but would "get an explanation and see that you get it."

When the explanation didn't arrive, the reporter again called Nix in Philadelphia and asked: "Will you tell me what happened to the $11,100 worth of stamps?"

"I have no explanation for you for what I do with my business . . . what I did is in accordance with my own duties and my business," Nix said.

Asked whether the stamps were used for congressional duties, Nix snapped, "I told you all that I have to tell you" -- and hung up.

Similarly, former representative Robert L. Leggett (D-Calif.) refused to tell what he did with $13,000 worth of stamps charged to his official expense account during his lame-duck period -- including $3,000 in postage obtained on his last day in office.

Leggett, now a lobbyists for maritime interests, explained that he had "lots of correspondence." But he wouldn't say what kind of correspondence it was, why he didn't use the frank, how many stamps he has left or why he never needed so many stamps before.

Finally, Leggett was asked: "Are you saying that this is your business and nobody else's?"

"That's right," the eight-term former congressman replied.

One former House member, who asked not to be identified, said it's "a common House practice" for departing House members to take some of their unused expense funds in stamps because they no longer can take it in cash.

"The stamps are as good as money," he said. "They just find someone who will cash them in."

The average expense allowance for a House member is $75,000 a year. The annual maximum amounts range from $59,810 to $147,000, depending upon the distance from a congressman's district to Washington and the costs of renting a district office. Thus, the House members from Hawaii, having the highest travel and long-distance telephone expenses, get the largest expense allowance.

The general rule about expense money is "use it or lose it."

Charles Howell, associate counsel of the Committee on House Administration, said neither his committee nor the House ethics committee is authorized to find out whether the stamps were used for official, personal or campaign purposes. The committees have no jurisdiction over former congressmen.

"We have to rely on the integrity of the member," Howell said. "In the absence of a large auditing staff, there's no way the committee can resolve the kinds of questions you are asking."

The administration committee's staff director, William G. Phillips, said he thinks that lame-duck congressmen are taking advantage of a loophole in the House regulations.

"About five years ago," he said, "Congressman H. R. Gross, the great economy advocate from Iowa, drew out a record amount [from his expense allowance]. It was something like $23,000 [in cash from the stationery account].

"There were all kinds of stink raised about it, and the regulations were changed."

House members no longer can draw out surplus funds in cash.

But, Phillips said, a congressman still can withdraw the excess in stamps "if he certifies it's for an official purpose."

It's "a loophole some of us are concerned about," he said. "We had proposed revisions to the regulations last spring which would have gotten at this stamp problem. The committee considered them and turned them down."

Meanwhile, he added, "We don't judge ethics here."

One former judge of ethics was former Rep. John J. Flynt Jr. (D-Ga.). He headed the House ethics committee but decided in the spring of 1978 to retire after 24 years as a congressman.

He used part of his expense account for $3,150 worth of stamps in the last three months of his political career.

The purpose, he said, was "mostly to send 'thank-you' letters to people who had been friends and supporters and so forth . . . I don't think anything like that should go under the frank. It's like Christmas cards."

James J. Delaney, who represented a Queens, N.Y., district for 32 years and became chairman of the House Rules Committee, didn't use any stamps in the entire year of 1977 but obtained $8,000 worth from the House postmaster in the closing weeks of his career.

He said he took the stamps with him into retirement in Key Biscayne, Fla.

How do you use $8,000 worth of stamps? he was asked.

"I don't know how you use them but I use them," Delaney replied. "I mean you've got to send out all your Christmas cards and you've got to send out all that other stuff.

As far as constituents are concerned, I've got to let them know they have got somebody else there [in Congress]. I tell them I'm no longer representing them."

And Delaney, though no longer a congressman, said he plans to use some stamps for the next Christmas, too, as "sort of a farewell."

Other former congressmen offered these explanations:

Fred B. Rooney (D-Pa.): He said he used $4,000 worth of stamps to answer mail from constituents "and I'm still using them."

Newton I. Steers Jr. (R-Md.), who was defeated last November and charged $2,000 worth of stamps on the last day of his term, said he used them to send out questionnaires on various public issues.

He said he "figured that if I put stamps on the mail, it costs the taxpayers absolutely nothing." He said the money went "from one government pocket to another" and helped "offset the Postal Service deficit."

He said he wanted to "let people express themselves" even though, when the answers to the questionnaires came back, he was no longer in a position to affect public policy. But, he noted, "I have filed to run for my old seat."

Rep. John Young (D-Tex.), whose office account was charged for $3,500 postage on the last day of his 11th and final term, said:

"I don't know anything about it . . . I can't imagine what I'd be getting stamps for . . . I appreciate your bringing this to my attention. I'm very curious about this myself. I'm damned sure gonna find out.

"What a finale, huh?"