Former representative Joseph P. Kolter (D-Pa.) admitted yesterday that during his 10 years in Congress he walked into the House Post Office, put thousands of dollars in taxpayer money into his pocket and lied on vouchers that said he had gotten stamps instead.

Kolter, 69, who served from 1983 to 1993, pleaded guilty to one count of conspiring with the former House postmaster, Robert V. Rota, and others to steal from the post office.

By doing so, Kolter deprived citizens of "their right to have the business of the U.S. House of Representatives conducted honestly and impartially, free from corruption, conflict of interest, deceit and fraud," prosecutor Wendy L. Wysong said.

Originally, Kolter was charged with embezzling more than $40,000 in taxpayer funds: $11,000 from the post office and more than $33,000 in merchandise from the House Stationery Store. He pleaded guilty to only one charge yesterday, involving seven incidents totaling $9,300 in thefts from the post office.

He is the first former congressman to admit that such a fraud existed at the post office. Former House Ways and Means Committee chairman Dan Rostenkowski (D-Ill.) vehemently disputed similar charges brought against him, even after he pleaded guilty last month to taking merchandise from the House Stationery Store and giving it to friends and using government employees to do personal chores, such as take pictures at his daughter's wedding.

Still outraged by Rostenkowski's combative remarks -- that his crimes were meaningless because he had simply done what everyone else in Congress had done -- prosecutors said Kolter's admission proves there was a cash-for-stamps scam in the post office.

"This demonstrates that Robert Rota was giving cash to congressmen," said prosecutor John M. Campbell, head of the public corruption unit in U.S. Attorney Eric H. Holder Jr.'s office.

"We were prepared to prove it," said prosecutor Thomas J. Motley. "We would've proven it."

Prosecutors said the post office investigation is continuing. Rota and two other post office employees who cooperated in the probe are awaiting sentencing; no dates have been set.

U.S. District Judge Norma Holloway Johnson set Kolter's sentencing for July 31. While the maximum penalty he faces under law is five years, his sentence under federal guidelines will probably fall within six to 12 months, lawyers for both sides said.

Wysong said Rota, Kolter and "other selected congressmen" set up a system that was "convenient and largely untraceable" to steal funds from the post office. They did it, she said, in two ways: Kolter would ask for and get a large number of stamps through a signed voucher and then, a short time later, take them back and exchange them for cash. But most often, Rota simply gave Kolter cash directly after he signed a voucher on the spot falsely stating that he had gotten stamps.

Kolter and Rota kept the conspiracy secret by having Kolter go personally to the post office to handle the transaction, Wysong said. Rota also kept a computerized list that kept track of the volume of stamps purchased by members of Congress. The two men often checked Kolter's purchases to make sure they were not out of line with the others so they would not draw attention, Wysong said.

After Wysong finished her summary, Judge Johnson asked Kolter if he agreed with it. The former congressman slowly shook his head. "Yes, ma'am," he said softly. She cautioned Kolter that he must abide by terms of his bond and asked him if he would return to court for sentencing.

"Looking forward to it," he said. CAPTION: JOSEPH P. KOLTER