At least part of $25,000 in Senate reimbursements to Sen. Herman E. Talmadge (D-Ga) for unauthorized or nonexistent office expenses could have ended up in Talmadge's personal bank account in 1975, the senator's private secretary acknowledged yesterday.

Allyne Tisdale, who has worked for Talmadge since 1966, told the Senate Select Committee on Ethics that the money was paid by the Senate disbursement office for noneixstent expenses claimed for Talmadge's Georgia office. The cash was mixed with some of Talmadge's legitimate funds and converted into bank certificates of deposit in a special account in Georgia that was maintained by Talmadge for both personal and office expenses, Tisdale said.

In Auguest 1975, she said, $80,000 from the certificates of deposit was transferred from the special account to another account in Talmadge's name at The Trust Co. of Atlanta. The Trust Co. account was solely for Talmadge's personal use, Tisdale testified.

The Senate Ethics Committee is holding hearings on five allegations of financial misconduct against Talmadge. One of the charges is that Talmadge knew of vouchers submitted to the Senate for office expenses that were never incurred.

Talmadge has not been accused of personally profiting from the Senate reimbursements. However, Tisdale's testimony is the first on-the-record indication by anyone connected with Talmadge that some of the money from the Senate overcharges ended up in the senator's account.

Talmadge's press spokesman and attorney told reporters after Tisdale's testimony that the only issue was not where the money went but whether Talmadge knew of the overcharges when they were made. Talmadge has denied any such knowledge.

Last year, after news accounts of the overcharges, Talmadge ordered an audit of his office finances. After the audit turned up the overcharges he repaid $37,125 to the Senate. Talmadge claimed the overcharges were due to confusion by his office staff and not deliberate.

Only an hour before Tisdale's testimony yesterday Talmadge left the hearing room and angrily told reporters outside the issue of the $80,000 transfer was "a cheap shot" raised Tuesday by committee special counsel Carl Eardley.

Waving a copy of an Atlanta newspaper headline about the transfer, Talmadge said the money was "personal funds accumulated over several years" and that there was nothing sinister about the transfer.

However, when Eardley sought information from Tisdale about the source of the $80,000, she was obviously reluctant to answer.

After three tries at getting Tisdale to discuss the source of the money for the $80,000 certificate of deposit. Eardley asked the Senate aide, "You mean some of it was generated from the over reimbursement?"

"You could say that could account for some of it," Tisdale replied.

According to Ethics Committee records, the Senate supplied $50,020 to Talmadge's office in over reimbursements between 1972 and mid-1978. About $25,000 of that money was paid to Talmadge by the time the $80,000 was transfered to his personal account.

Earlier another Talmadge aide, Rita Hubler, told the ethics panel that Talmadge kept personal funds as well as senate reimbursements in the special office account in Atlanta because the account occasionally needed extra money to meet office business expenses.

Hubler, who kept Talmadge's books during 1975, said she "refreshed my memory" Tuesday night about details of the $80,000 transfer from the office account to Talmadge's account. Hubler told the committee Tuesday that she had no recollection of the transaction.

The money , she said yesterday, came from such sources as bank dividends, interest and honoraria and all belonged to Talmadge. Hubler said interest payments on Talmadge's personal portion of the office account were then used to pay office expenses when the regular office fund ran low. "It was idle money put to work," she said.

Hubler did not explain, when she was asked by members of the Ethics Committee, why the senator's personal funds went into the office account in the first place.

It was also unclear from her testimony why the office account would have needed the interest on Talmadge's personal funds. Aides to the senator have testified that the office was reimbursed $40,000 annually by the senate, well over the amount it actually spent.

Yesterday's proceedings ended in a dispute between Eardley and Talmadge's attorney. James Hamilton. When Eardley protested Hamilton's motion to exclude key documents obtained by former Talmadge aide Daniel Minchew, the committee adjourned. They later agreed to review the documents on a day-by-day basis. CAPTION: Picture, Sen. Talmadge confers with his attorney, James Hamilton, during ethics hearing. By James K. W. Atherton-The Washington Post