Polyamines are thought to play an essential role in cellular hypertrophy and proliferation. Ornithine decarboxylase (ODC) catalyzes the first and probably the rate-limiting step in biosynthesis of polyamines. In this study, we evaluated the pathophysiological role of the renin-angiotensin system in isoproterenol-induced cardiac hypertrophy, using myocardial ODC activity as an indicator of cardiac hypertrophy. Isoproterenol caused an eight-fold increase of myocardial ODC activity in normotensive Wistar rats within 4 h after injection. Captopril suppressed the induction of ODC by isoproterenol to two-thirds of the control level. These results indicate that the renin-angiotensin system may participate in the induction of myocardial hypertrophy by isoproterenol.