The release of endorphins appears to play a role as a reserve-mechanism in stress situations. 2. The application of high doses of the specific opiate antagonist 12-allyl-7,7a,8,9-tetrahydro-3,7a-dihydroxy-(4aH)-(8,9-c)-iminoethanophenanthro(4,5-b,c,d)furan-5(6H)-one (naloxone) to schizophrenic patients induces a partial reversal of hallucinations and actual delusional experience within 2--7 h after injection. 3. The blockade of opiate receptors by naloxone is possibly also effective against catatonic states of schizophrenia. 4. In affective psychoses no curative action of naloxone could be detected so far.