A nurse who helped Dr. Robert J. Sherman conduct abortions at his Northwest Washington clinic testified yesterday that she quit her $180 a week job after 27 days because she felt she would be unable to bring Sherman's medical practices, which she called "obnoxious," up to safe standards.

During almost six hours of testimony in D.C. Superior Court. Alexis Morman, 30, told the jury she thought Sherman's clinic was dirty, that Sherman, who performed five to 15 abortions a day, was "always rushed," and that unclean, broken and worn instruments were used on patients to cut clinic costs.

Morman also told the jury, under cross-examination by defense attorney Robert F. Muse, that while employed by Sherman she allowed him to fit her with a birth control device. She agreed to the procedure, she later testified, because of personal financial pressures and complications with another birth control method.

"I was desperate," Morman told the jury.

Morman completed two days of testimony yesterday as a government witness at Sherman's trial on charges of second degree murder and 26 counts of perjury.

Sherman is charged with deliberately performing an incomplete abortion in March 1975 on 16-year-old Rita C. McDowell - a procedure that, according to the prosecution, resulted in her death four days later. The government contends it will prove that Sherman forged and altered documents and induced employes to lie for him to cover up practices at the clinic and the circumstances of McDowell's death.

The prosecution told the jury it would prove that Sherman acted in malicious disregard for his patients' safety in order to increase his clinic's profits.

In her testimony for the government, Morman said Sherman performed abortions at his clinic on women who were more than 12 weeks pregnant, which the government contends violated medical standards. Morman testified that half of the abortion patients paid for he precedure through insurance.

During four hours of cross-examination yesterday, Morman said she saw 200 to 300 abortions at the clinic during her employment there, including one conducted on a 13-year-old girl who was five months pregnant. Morman worked for Sherman in September and October, 1974.

Defense attorney Muse repeatedly challenged Morman's ability to recollect details about procedures at the clinic four years ago. Muse also questioned Morman's failure to file a formal complaint about Sherman's abortion practice until June 1975, eight months after she quit her job at the clinic.

Morman testified that she had attempted to bring a complaint earlier, but was unable to determine from city officials where such a complaint should be filed. In May 1975, Morman tetified, she met with Washington lawyer Aaron M. Levine, who sued Sherman for malpractive on behalf of Rita McDowell's mother and young son. The family eventually collected more than $500,000 in damages.

A month after she met with Levine, Morman testified, she filed a formal complaint with the D.C. Medical Society.