WASHINGTON (AP) _ The Supreme Court today refused to free some states from having to pay for a hypnosis expert when a penniless criminal defendant challenges a prosecution witness's hypnotically enhanced testimony.

The court, without comment, rejected an appeal by Missouri prosecutors seeking to reinstate the rape conviction of a man who was identified by the victim after she was hypnotized.

Leatrice Little was convicted of rape and sentenced to 25 years in prison for attacking a woman in her Cape Girardeau apartment Aug. 13, 1980. He has since been freed on parole.

The victim said she saw the face of her assailant briefly, when a blouse he had taken from her closet and used to cover his face slipped for at least a few seconds.

After police hynotized the victim, she identified Little from a group of photographs. At a subsequent police lineup and again at the trial, she identified Little as her attacker.

Little said he was too poor to pay an expert to challenge the reliability of the hypnotically enhanced testimony. He said an expert might show that the victim had been given improper suggestions under hypnosis.

The 8th U.S. Circuit Court of Appeals last December ruled that Little was denied a fair trial, and it overturned his conviction.

''The denial of a state-provided expert on hypnosis to assist this indigent defendant rendered the trial fundamentally unfair and requires that the conviction be set aside,'' the appeals court said.

Its ruling was binding on all courts in Missouri, Arkansas, Iowa, Minnesota, Nebraska, North Dakota and South Dakota.

But hypnotically enhanced testimony was ruled inadmissible in Missouri courts some time after Little was convicted.

The appeals court noted that the Supreme Court last year ruled that all hypnotically enhanced testimony offered by a criminal defendant may not be excluded automatically. But that ruling did not deal with hypnotically enhanced testimony by prosecution witnesses.

The appeals court said if such methods for obtaining testimony are to be used, poor defendants should not be placed at a disadvantage.

The appeals court noted that the Supreme Court in 1985 ruled that states must pay for psychiatric experts for a defendant in a capital trial when the defendant's sanity is a significant issue.

''There is no principled way to distinguish between psychiatric and non- psychiatric experts,'' the appeals court said. ''The question in each case must be not what field of science or expert knowledge is involved, but rather how important the scientific issue is in the case and how much help a defense expert could have given.''

The appeals court said Little ''demonstrated that an expert in hypnosis would have substantially aided his defense.''

The case is Armontrout vs. Little, 87-1532.