Last March, Rep. Larry Pressler (R-S.D.) spent public funds from his official House expense allowance to buy time on television stations in his district and air a number of 30-second and 60-second spots that showed him discussing a controversial federal water project, a Labor Department employment report and federal job safety regulations.

The official expense allowance, according to present House Administration Committee regulations, is to be used for "ordinary and necessary business expenses." But for a congressman, what is an ordinary business expense and what are political or personal expenses?

Pressler said last week he has a letter from House Administration Committee Chairman Frank Thompson Jr. (D-N.J.) okaying use of official allowance money for buying time for the spots.

He said he spent $2,000 from his official allowance on these "television newsletters," which he described as "straightforward, nonpolitical." He acknowledges, however, that they were produced by the National Republican Congressional Committee and paid for out of campaign funds.

The House Administration Committee apparently approved a wide variety of expenditures from each member's $2,000-a-year official expense allowance, including bar association dues, country club assessments, flowers, liquor, entertainment, theater tickets, charity donations and, in one case, staff help despite the fact there is a special and limited clerk-hire fund.

Since the clerk of the House report disclosed recently a range of questioned "official" expenditures, Thompson's Administration Committee has decided to take another look "with an eye to amending existing regulations to provide more guidance," according to an aide.

The main focus, the aide said, would probably be on prohibiting use of the allowance for "dues to nonlegislative groups, flowers, donations, mass cards." An attempt will also be made, he said, to "limit" spending on food and beverages, but strong opposition to that is expected.

While the committee examines these personal expense areas, purchases of television time and programming assistance open an even wider area for inquiry.

In May, for example, Rep. Larry Winn Jr. (R-Kan.) spent $260 in tax money for a Washington television camera crew to record and edit his monthly 30-minute TV program which is shown over Kansas City's public television station. This one featured the congressman interviewing Michael Collins, the former astronaut, at the Smithsonian Air and Space Museum, where Collins is now director.

In the past, according to Winn's press secretary, Meredith A. Masoner, funds to pay for the monthly program came from the GOP House campaign committee.

That committee, which used to give each GOP member an annual $2,000 to $3,000 public relations supplement - paid for by campaign funds - now does some of the same work, but gets reimbursed by the member from an official allowance.

The committee, for example, has its film crew which is provided to shoot spots outside the House film studio to members at cost. Several made use of the crew and reported using tax dollars from their official expense allowance to pay the committee for the service. In the pre-reform days, the committee would have financed the same service from its campaign funds.

Television is not the only area open for exploration.

Rep. G. V. (Sonny) Montgomery (D-Miss.) each year uses tax money to purchase engraved plaques that are awarded to top Boy Scouts and Junior Naval ROTC students in his district.

A spokesman for Montgomery said the ROTC awards are made because the congressman helped get the high school programs started and they have "fostered ties with the schools and the Navy Base" in Meridian - a key facility in the district.

As far as the awards having political benefit, the aide said he saw none because "frankly we don't get much publicity out of it."

Award giving, however, is widespread among members and considered a campaign gimmick by several - who don't want to be quoted.

Rep. James M. Hanley (D-N.Y.) has just started giving out about 50 "Congressman's Medals of Merit Awards" to high school students selected by their schools for achievement in citizenship.Hanley's medal idea, which cost taxpayers $157.51, was picked up this year from several other New York legislators. Hanley thought it "worth trying," an aide said.

Was it political? Hanley's aide said it was "something the schools wanted," but added, "Anything a member does is political."

Rep. Thomas Downey (D-N.Y.) had a program of giving plaques to individuals who helped with his mobile office programs - buying the awards with his official allowance. He said yesterday he had stopped the program because he came to question whether it was a "legitimate" official expense.

It was Downey, however, who used $1,050 from his expense allowance to hire a consultant to help him with his duties as a member of the Board of Visitors to West Point. Downey had intended to charge the fee off to his clerk-hire account but his 13 slots - all that are permitted - were already filled.

House Administration, according to Downey approved use of expense funds for the consultant who, as a former West Point faculty member, helped Downey with his visits to the academy and his final report.

House reforms over the past few years have greatly changed the House allowance system - mainly by raising them and by permitting their free and easy use for anything a member thought he could defend with his constituents.

Newsletters and polls and radio and television spots were once paid out of ried in unreported, and therefore unofficial, office accounts. Those members who had surplus campaign funds also used them for such items and others, such as flowers and charity donations, that have traditionally been the stock and trade of politicians.

A new "constituent communications" allowance of $5,000 was approved in 1975 to allow every member to send "nonpolitical" newsletters.

In the wake of the Wayne Hays scandals, the allowance system was changed to permit funds to be transferred from one category to another. At the same time some were increased.

When it was decided to wipe out office accounts and require reports on spending of official expenses, it was also agreed to give each member an additional $5,000 beginning next year - an election year.

More recently, the home office rental allowance was raised - so high that many members will be eligible for more money than they need. That money can be transferred to other accounts to increase nonpolitical television, radio and mailings next year.

"Every time they tightened down on reporting," a key House staffer said, "they gave them more money to sweeten the pie."

Downey, a Democrat elected in a 2-to-1 Republican district, said recently that the "allowances are reasonably lavish . . . and give incumbents a tremendous advantage."

They lead to strange habits. Downey does not use his expense allowance to rent a car in his district to go from meeting to meeting.

His reason: 'Events are both political and official. How could I sort them out?"

Another young Democrat, Rep. Paul Tsongas (D-Mass.) pays for his mobile office out of surplus campaign funds.

The official expense reports shows many members charge their official accounts for leasing cars used in their districts.

"No one wants to say what is legitimate for a member of Congress to do with official funds," Steven F. Stockmeyer, executive director of the National Republican Congressional Committee, said recently. "They can't determine where you cross the line" between official and political expense.

Stockmeyer should know. He is trying to spend his organization's funds for political expenses only - because of International Revenue Service rulings - but many of those things he used to do for members are now considered "official" thanks to the new reforms.