When Joel L. Merling learned that his 87-year-old mother was critically ill in a Canadian hospital with pneumonia and acute leukemia, he sought an emergency furlough from the federal prison at Lexington, Ky., where he is serving a five-year sentence on a narcotics conviction.

The federal Bureau of Prisons was well aware of the "evident humanitarian concerns" in Merling's case, and his desire to be at his mother's side, according to records in U.S. District Court here. But it denied the 45-year-old Canadian citizen's plea for a furlough -- a brief release from prison -- saying that it did not have the authority to grant a request for a furlough to a foreign country.

Merling's lawyer, Brian W. Shaughnessy, took the case to the federal court here. This week, in an unprecedented decision, Judge Gerhard Gesell reversed the bureau's 15-year-old policy on such requests and said it had the discretion to grant a furlough that would allow the prisoner to visit Canada.

Gesell noted in a written opinion that when Congress gave the bureau furlough authority in 1965, the policy was designed to help prisoners develop "constructive family relationships." The only limit on the furloughs, Gesell wrote, was that the bureau believed that the prisoner was trustworthy and that the visit would serve purposes set out in the furlough program. The bureau's refusal to exercise any discretion in deciding furlough requests to Canada contradicted the intent of the program. Gesell concluded.

A spokesman for the Bureau of Prisons said that more than 20,000 furloughs are granted annually to about 4,000 federal prisoners, most of whom have nearly finished their prison terms.

Furloughs can be granted for from one to 30 days