animals that received the monoamine oxidase inhibitor and the a-methyltyrosinecontaining diet, motor activity and brain amine levels were lower than corresponding control values. However, values for the spontaneous locomotor activity and the brain content of noradrenaline were significantly higher in the pheniprazine-a-methyltyrosine group than in saline-cr-methyltyrosine group. Although pheniprazine partially blocked the behavioural depression and noradrenaline depletion it did not alter plasma levels of a-methyltyrosine. Thus the behavioural depression after a 24 hr diet containing a-methytyrosine appears to be related to the brain levels of catecholamines. This work was supported by USPHS Grant AM-1 1083. L-a-Methyltyrosine was kindly supplied by Dr. C. A. Stone, Merck Institute for Therapeutic Research, West Point, Pa., U.S.A. Pheniprazine (JB-5 16, Catron) was supplied by Dr. R. C. Ursillo, Lakeside Laboratories, Milwaukee, Wisconsin, U S A . The technical assistance of Mrs. M. Grarnatins and Mrs. D. Simpson is gratefully acknowledged.