A letter that John W. Hinckley Jr. wrote to obtain a nude sketch of actress Jodie Foster scuttled a mental hospital's plan to take the presidential assailant out on a field trip, according to sources familiar with the case.

Hinckley's correspondence with a mail-order house was discovered by the Secret Service after prosecutors found what they termed "disturbing entries" in Hinckley's treatment record at St. Elizabeths Hospital, according to sources who spoke only on condition they not be identified.One source said Hinckley obtained the drawing of Foster in the nude.

A recurring theme in Hinkley's case has been his obsession with the actress, who played the role of a prostitute in the movie "Taxi Driver," which concerned a plot to kill a senator seeking the presidency.

Prosecutors contended that Hinckley shot President Reagan and three other men here on March 30, 1981, to impress Foster. Hinckley, now 33, was acquitted by reason of insanity and was committed indefinitely to the Washington mental hospital.

Last year, pictures of Foster were found in Hinckley's dormitory-style room at St. Elizabeths before hospital officials abruptly withdrew a request to allow him to visit his parents over the Easter weekend.

The search was ordered by a judge after a psychiatrist, Glenn Miller, disclosed in testimony that Hinckley had written to multiple murderer Theodore Bundy on Florida's death row.

During last year's court proceeding, Miller testified that Hinckley was no longer obsessed with Foster, about whom he had made daily statements during interviews five years earlier.