A mental hospital holding John W. Hinckley Jr. on Friday dropped its proposal to take the man who shot President Reagan on a field trip, as prosecutors turned up new writings by the presidential assailant.

Dr. Raymond F. Patterson, director of St. Elizabeths Hospital's forensic services unit, told a federal judge that the decision to drop the request was based on material received by the hospital Thursday night.Reading a letter to U.S. District Judge Barrington D. Parker, Patterson said, "The Forensic Services Administration has determined that further clinical assessment by Mr. Hinckley's treatment staff is necessary before it would permit him to participate" in a daylong field trip.

Comments by attorneys on both sides during the brief hearing indicated the material turned up by the Secret Service consisted of letters by Hinckley to an unidentified third party.

U.S. Attorney Jay B. Stephens asked Parker to unseal the material, which he said the Secret Service had obtained through an investigation. Defense lawyer Vincent Fuller described the material as letters written by Hinckley.

Parker refused to unseal the writings, which were turned over to the court Thursday, until Fuller could respond to the government's request they be made public. The defense lawyer was given until Tuesday to file a written objection.