Gossypol was found to have an inhibiting effect on spermatogenesis in animals and has been used as an antifertility agent in clinical trial. However, a very small proportion of subjects on gossypol were found to be hypokaleimia. The direct cause of it was due to renal potassium loss. The present study was designed to make an ultrastructural observation on the enzymatic activity of the membrane protein Na-K-ATPase, the so called sodium pump of renal tubular cells of rats and guinea pigs following gossypol treatment. Gossypol was administered to male animals at a dosage of 30 mg/kg body weight daily. Kidney tissues were removed at various time intervals after gossypol treatment and processed for ultra-cytochemical study and quantitative measurement of Na-K-ATPase. The results obtained were as follows.1. The enzymatic activity of Na-K-ATPase of kidney tissues was shown to decrease slightly following gossypol treatment in both rats and giunea pigs, but there was no significant difference between the gos ol treated and the control groups.2. Ultra-cytochemical observations on the localization of Na-K-ATPase demonstrated that the enzyme reactive products in renal distal convoluting tubules, ascending limbs of Henle's loop and collecting ducts desending through the cortex to the outer medulla were mainly localized to the basal plasma membranesand their retromembranous folds of the epithelial cells. A small amount of the reactive products associated with the luminal surface of the distal convoluting and collective tubules were also observed. The amount of enzyme reactive granules of the treated animals did not differ significantly from that of controls.3. The results of perfusion experiments with ouabain indicated that ouabain inhibited Na-K-ATPase specifically in renal distal convolutions, ascending limbs and collecting tubules in both the treated and control animals.4. The cytochemical reactive granules in the erythrocyte and cardiac cell membrenes decreased in number following gossypol treatment.