The objective of this study was to compare the effect of zinc deficiency on the activity of angiotensin-converting enzyme (ACE) in lung and testis of rats. Two groups of 32-d-old male albino rats were fed ad libitum or pair-fed a low zinc (less than 1 mg/kg) diet based on soybean protein. Two other groups were fed ad libitum or pair-fed a similar control diet containing 50 mg of zinc/kg diet. After 26 d, lungs and testes were removed and ACE activity was determined in their homogenates. Low food intake, reduced growth rate and low serum zinc concentration demonstrated that the rats were zinc deficient. Testicular ACE activity in the zinc-deficient rats was significantly lower (P less than 0.001) than that in the zinc-adequate controls. On the other hand, lung ACE activity of deficient rats was significantly higher (P less than 0.001) than that of controls. Pair-feeding had no significant effect on ACE activity in testes, but lung ACE activity was significantly higher (P less than 0.001) in pair-fed than in ad libitum-fed controls. Pair-feeding of the deficient diet had no effect. Low ACE activity in testes of zinc-deficient rats may be the result of under development of the germinal epithelium. The cause of elevated ACE activity in lungs of deficient rats is not apparent.