Cat left ventricle heart slices and brain cortex slices and homogenates were employed in the Warburg manometric technique to examine the influence of chlorpromazine on their aerobic metabolism. Concentrations of 10-20 mg% chlorpromazine required to suppress their O2 consumption were considerably greater than those that evoke a response in vivo and in other in vitro experiments, 0.025-0.5 mg%. It appears unlikely that the harmacological actions of the compound are due to an effect on oxidative metabolism.