Ultrastructural changes in the region adjacent to the nuclear pole were identified in the ventricular myocardium of ouabain-treated guinea pigs after 1 to 4 hours of treatment. Significant changes include expansion of the Golgi complex, an increase in the number of coated vesicles and an increase in the number of cytoplasmic particulates including glycogen particles and ribosomes. The coated vesicles appear to coalesce with the internal terminal regions of T-tubules and thereby may contribute to increase tubular mass or surface. In contrast to acute ouabain treatment, the morphologic change identified with the administration of another positive inotropic drug, isoproterenol, consists of some increase in cytoplasmic particulates, but no significant change in the Golgi apparatus or in the number of coated vesicles compared with control hearts. It is concluded that the administration of ouabain leads to activation of the Golgi complex, possibly associated with the synthesis of new T-tubular membrane components.