A drug-induced change in the fine morphology of the plasma membrane of heart muscle cells is described. Thus, in rabbits the chronic administration of oxyfedrine caused a marked proliferation of the plasma membrane. At the same time the mitochondria became swollen and vacuolated, and there was evidence of lysis of myofibrils. These changes are not accounted for in terms of the release of endogenous stores of catecholamines.