Mary Treadwell took the witness stand at her fraud and conspiracy trial yesterday and firmly denied that she ever took any money from her firm's operating account on the dilapidated Clifton Terrace apartment complex for her own use or for her various business ventures.

Treadwell, testifying in a loud and confident voice for 40 minutes in a hushed federal courtroom, acknowledged, however, that on at least four occasions in the mid-1970s she found that money from the P.I. Properties Inc. account for Clifton Terrace had been used for expenditures elsewhere.

She said that she reimbursed the account and blamed the firm's former general manager, Robert E. Lee, for improperly handling the money. Lee has already pleaded guilty to conspiracy in the case.

Treadwell also told the jury of eight women and four men that she has "a habit of filing federal income tax returns late" and admitted that she had only paid $18,800 in federal taxes for the years 1976 to 1978, three years for which prosecutors contend she evaded another $33,000 in taxes.

But Treadwell, answering questions from John W. Nields, one of her court-appointed attorneys, said she declared all of her income for those years and made no effort to conceal any income. She filed the 1976-78 returns in 1980.

"I had reasons for filing late , but no excuses," she said.

Treadwell is accused of using P.I. Properties to defraud the federal government and the impoverished tenants at Clifton Terrace of thousands of dollars to enrich herself. Nields asked her about her version of a series of financial transactions that the government contends are examples of her use of the company accounts as "her own personal piggy bank."

Her defense attorneys have tried repeatedly to blame Lee and Treadwell's sister, Joan M. Booth, Clifton Terrace's project manager, for any wrongdoing involving P.I. Properties' accounts.

Toward that end, Charles W. Rinker Jr., the defunct P.I. Properties' former secretary-treasurer and the only person charged and then cleared in the case, testified yesterday that Booth told him in a chance conversation in April 1982, two months after they had both been indicted, "that Mary and I were the only innocent parties in the case."

Rinker, a 42-year-old Methodist minister, recalled Booth telling him in the conversation at the Judiciary Square subway station that "she felt sorry for what she had done" and that "she was stupid." Rinker also quoted Booth as saying that "Bob Lee had lied to all of us, including her, for four years" about his involvement in the case.

Assistant U.S. Attorney Stephen R. Spivack attempted to prohibit Rinker from testifying about the conversation with Booth, but U.S. District Judge John Garrett Penn ruled it admissible after lengthy bench conferences on the issue.

Spivack did not cross-examine Rinker about his conversation with Booth. But in lengthy questioning, the prosecutor got Rinker to concede that by late 1976 and into 1977 he "did not know much of what was going on at Clifton or P.I."

In her testimony, Treadwell said she was "not aware" of three checks that Lee wrote out of the Clifton Terrace account. One of the checks, for $8,900, helped pay for the purchase of a house that another Treadwell venture, Sticks and Stones Inc., rehabilitated and sold. Treadwell said she received no money from the check or the sale of the property and repaid the check to Clifton Terrace out of the Sticks and Stones account.

In another instance, Treadwell said she reimbursed Lee for decorating work he had done at her Kiosk advertising agency on Connecticut Avenue NW. Treadwell said she rejected a second claim Lee made that she had not paid him for the work.

The prosecutors have claimed that Treadwell illegally wired her fiance, Ronald S. Williams, $3,500 in P.I. Properties funds in August 1977 so that he could use the money to buy her a wedding ring. While some of the money was used to buy the ring, Treadwell, now divorced from Williams, adamantly testified that her former husband had earned the money when he started working on an audit of P.I. Properties' books. The audit, however, was never finished.

"Was he owed that money?" Nields asked Treadwell.

"Yes, he was," she replied.