Changes in systolic blood pressure (SBP), heart rate (HR), heart rate and plasma renin concentration (PRC) have been compared in three different groups of rats between the ages of 5 and 20 weeks. The groups were: spontaneously hypertensive rats (SHRs), atenolol-treated SHRs (200 mg/kg/day orally throughout the 15 weeks) and normotensive rats of the same strain (WKYs). Treatment with atenolol markedly inhibited the onset of genetic hypertension, reduced HR and PRC from the outset and diminished the heart weight/body weight ratio. Comparison of changes in these parameters in atenolol-treated SHRs, control SHRs and WKYs strongly suggests that the mechanism of atenolol's preventive action against hypertension development in SHRs primarily involves its effects on heart and on the renin--angiotensin system.