Gonad, lung, kidney and serum angiotensin converting enzyme (ACE) activities were determined by specific substrate hydrolysis in male and female Rana esculenta over 1 year. Ovary ACE activity showed the highest values among the different tissues, with a significant peak (223+/-52 nmol min(-1) mg protein(-1)) in late winter-early spring. Testis ACE activity followed a significant seasonal cycle, increasing from September to peak in April (2.5+/-0.8 nmol min(-1) mg protein(-1)) and then decreased in the post-reproductive period. Lung and kidney ACE activities were not correlated with the annual reproductive cycle phases. In serum a peak of activity was present in the post-reproductive period both in male and female frogs. The present data show a correlation between ACE and the annual reproductive cycle of R. esculenta.