The effect of the administration of p-chlorophenylalanine on the fine structure of the ventricular cardiac muscle of active and hibernating bats was studied. The concentration of norepinephrine in the bat heart was also measured. In the active animal, p-chlorophenylalanine induced a striking increase in the size and number of lipid droplets. Moreover, the lipid droplets exhibited an intimate association with mitochondria. The mitochondria in such associations showed extreme variations in form but exhibited normal arrangement of cristae. p-Chlorophenylalanine also induced a slight increase in the number of glycogen granules but no other effects were apparent. During hibernation, the administration of p-chlorophenylalanine failed to produce a change in either the size or number of lipid droplets. The concentration of norepinephrine in the heart of active and hibernating bats was not altered by the injection of the drug. The present findings indicate that lipid accumulation in ventricular myocardium following injection of drugs is not necessarily the result of depletion of cardiac stores of norepinephrine.