Following probenecid administration lumbar cerebrospinal fluid 5hydroxyindoleacetic acid (5HIAA) and homovanillic acid (HVA) were measured before and during phenothiazine treatment in 12 patients who developed psychotic episodes following the ingestion of a psychotomimetic drug, probably D-lysergic acid diethylamide (LSD-25) in all but two cases. Compared to nondrug-induced psychotic patients and inmate controls the drug-induced patients gave evidence of decreased central 5HIAA formation, findings analagous to those obtained following the administration of LSD-like drugs to animals. These results persisted during phenothiazine treatment.