Cycloserine appears to be a psychopharmacological agent in that 1. It reverses depressions to normal or hyperactive mood and at the same time causes subtle changes in behavior; 2. It may aggravate mental disorders; and 3. It seems to potentiate Marsilid. Cycloserine may become a valuable tool in psychopharmacological research and may also find therapeutic use in depressions, provided the initial dose does not exceed 500 mg. a day and potentially psychotic patients receive more than average supervision.