A dentist told a Montgomery County Circuit Court jury yesterday he gave a patient nitrous oxide gas at his Columbia, Md., office, causing fatal injury, thinking that he was administering oxygen.

The dentist, Saul J. Schweber, who lives in Silver Spring, testified that he had no way of knowing that nitrous oxide, an anesthetic also called laughing gas, was coming out of a valve in his office marked for oxygen.

His patient, Jody Lynn Nurik, a 22-year-old Catholic University student, died in Johns Hopkins University Hospital on March 20, 1974, four days after she was administered nitrous oxide for 8 minutes.

Attorneys for Mrs Nurik's husband contend that the anesthetic, which is lethal if inhaled straight, was coming out of the wrong value because of faulty installation of the piping system used for both gases in Schweber's office.

Marc Steven Nurik, 27, the dead woman's husband of one year, is suing Schweber for $8.5 million, Deeley Dental Supply Inc. of Baltimore, the company that Schweber's dental equipment; and Paul Freedman Inc., of Rockville, the plumbing firm that installed the gas pipes.

Under questioning by Nurik's attorney, Herman Glaser, Schweber acknowledged he never performed any tests on his equipment to make certain that the proper gas was coming out of the proper valve.

Schweber said his dental partner told him about three weeks before Mrs. Nurik's death that some patients "were not coming around quick enough" when they were given the nitrous oxide. These patients, unlike Nurik, were receiving does of nitrous oxide mixed with oxygen, Schweber said. Such does were safe.

The dentist testified that he twice asked the Deeley company to check out his equipment before the Nurik incident took place. Schweber said he continued to use the same equipment for a short time after Nurik's death because "I had no idea" about the mix-up with the gases.

He testified that he had thought that Nurik, who turned blue when he gave her the nitrous oxide, had suffered cardiac arrest in his dental chair as a reaction to the sedative and pain-killer he had injected. While in the dental chair, Nurik, who was having four wisdom teeth pulled, stopped breathing her heart stopped as well, Schweber said.