OURDANTON, Tex., March 29 — Hundreds of patients of a hospital in this quiet ranching community have been tested for H.I.V. after a nurse, herself infected with the virus, admitted stealing doses of a narcotic and possibly using infected needles to replace them with a saline solution, the authorities say.

The hospital, the South Texas Regional Medical Center, has advised 1,100 patients to be tested for H.I.V., which causes AIDS.

"We looked at all of the possibilities and our responsibilities, and that's what we acted upon," Allan Smith, the hospital's administrator, said. "The evidence is that it probably didn't happen, but we've decided to proceed with an abundance of caution and have asked patients who received Demerol during the period she worked at the hospital to have the tests."

Mr. Smith said that because H.I.V. lives so briefly outside the body, it is unlikely that any patients would have been infected, even if the nurse did use infected needles.

"The doctors we've talked to believe there is less than three-tenths of one percent chance that someone could become ill in this way," he said. "The possibility is just very, very remote."

No test results are complete for the 275 patients who have been checked, he said, but health experts advising Mr. Smith have suggested that a number of the 1,100 people notified of their risk by the hospital would test positive for the virus.

"That's just the ordinary rate of the disease in the population, and that won't have anything to do with what happened here," he said.

Hospital officials here said the nurse, whom they would not identify, had worked at South Texas Regional from June 4, 2001, until Jan. 4, 2002. She has admitted stealing the narcotic, Demerol, and replacing it with saline, but has no memory of whether she used infected needles during the process, they said.

Officials at the Texas Board of Nurse Examiners said the hospital had filed a complaint against Jacqueline H. Fillingim, 36, who received her nursing degree in 1990 from McClennan Community College in Waco.

Ms. Fillingim voluntarily surrendered her nursing license in October 1995 and was treated for chemical dependency problems before being reinstated in 1998, the officials said.

The hospital's complaint against Ms. Fillingim accuses her of withdrawing five doses of Demerol without a doctor's order, falsifying patient records, and withdrawing one dose of Demerol without giving it to a patient, according to documents obtained from the nurses board. The board has asked Ms. Fillingim to surrender her license.

"The facility has confirmed that the incident involved a nurse we were investigating," said Dusty Johnson, general counsel for the board, referring to the hospital.

Although the hospital has reported the incident to the Texas Department of Public Safety and the local sheriff's department, no criminal investigation is being pursued, officials at those agencies said.

Mr. Smith said hospital officials became suspicious about missing Demerol in late 2001, conducted an investigation and in early January confronted the nurse, who admitted her addiction.

Because Demerol is injected intravenously, the nurse was asked to take a blood test, which is when she told hospital officials she was H.I.V. positive, he said. Subsequent testing confirmed that she was infected, Mr. Smith said.

"We were unaware that this nurse had a prior record of drug problems," he said. "The Board of Nurse Examiners never told us."

Last week, the hospital began sending certified letters to potentially infected patients. It has received about 400 calls requesting more information or scheduling H.I.V. tests, said a hospital spokeswoman, Rosemary Walsh.