In cats decerebrated by a midcollicular section, decerebrate rigidity developed that was not alleviated by phenytoin even in doses as great as 60 mg per kilogram. Chlorpromazine that depresses decerebrate rigidity in a dose-related fashion requires 1.5 to 2.0 mg per kilogram to exhibit an appreciable effect. In the presence of 20 mg per kilogram phenytoin, however, as little as 0.1 mg per kilogram chlorpromazine markedly reduces decerebrate rigidity. This drug combination did not impair neuromuscular transmission nor did it severely impair motor coordination in cats. Although phenytoin depressed muscle spindle discharges, this peripheral suppression was insufficient to abolish the rigidity. Phenytoin with or without chlorpromazine may be of value in suppressing muscle rigidity in some disorders of upper motor neuron lesions.