A U.S. Naval Academy midshipman, one of only two midshipmen accused of drug use since regular urine testing was instituted two years ago, is fighting academy efforts to expel him.

Jeff Bellistri, a 23-year-old senior from Severna Park, Md., says he did not use cocaine and that he believes urine samples showing traces of cocaine must have been tampered with or mixed up. An academy spokesman said Navy officials believe the test was properly administered.

The last time a midshipman left the academy for drug-related reasons was in February 1984. In that case, which occurred before the academy instituted urine tests, the midshipman resigned after being charged with possession of marijuana.

Bellistri said he has been found guilty at two administrative hearings at the academy and that his case is being reviewed by Adm. Charles R. Larson, the academy superintendent.

Academy officials said the case of a second student accused of using cocaine is also under review.

Bellistri said academy officials told his attorneys today they would allow him to resign, releasing him from a requirement to serve out an enlisted term, if he agreed to the offer by Thursday morning. He said he has not decided whether he will accept the offer.

Regular urine testing of midshipmen was instituted during the past academic year, with officials testing about 25 students a month. Last summer, officials expanded the program to test about 250 students a week.

"Urinalysis is used throughout the Navy," said Cmdr. Stephen H. Clawson, a spokesman for the academy. "When graduates go to their ships or squadrons or Marine Corps units, they are going to be subject to [testing]. What we are doing is not inconsistent with Navy policy. That policy is zero tolerance of drug abuse."

Clawson said care is taken to ensure that urine samples are not lost or mixed up when they are sent to the Navy drug lab in Norfolk. "In this case of Midshipman Bellistri, we have investigated it very thoroughly and considered it very carefully, and given the midshipman the opportunity to represent his case."

Bellistri, who was tested on Nov. 12, said officials tested the urine sample a second time and evidence of cocaine was found again. He said he offered to give them a second sample but was told it was too late.

He said that at one of his hearings, it was disclosed that at least two samples had been mixed up on the day his sample was tested, although the error was corrected. Navy officials would not comment.

Bellistri said he passed a lie detector test arranged by his brother-in-law, an Annapolis lawyer, but Navy officials refused to accept the results. Navy investigators gave him a second lie detector test, he said, but found the results inconclusive.

Bellistri, who has hoped to become a Marine Corps pilot, said he has had no other academic or discipline problems at the academy. "I want to be an officer in the military," he said, "and they are kind of getting in my way."