The effects of chronic oral administration of bopindolol (twice daily for 12 weeks beginning at 5 weeks of age) on serum lipoprotein concentrations, blood pressure and heart rate, were studied in spontaneously hypertensive rats. They were compared with those of two other beta-blockers. Bopindolol and propranolol (15 mg/kg) attenuated the increase in heart rate. Pindolol produced a similar, gradual change in heart rate, but only after the 6th week of treatment. Propranolol and bopindolol attenuated the development of hypertension, whereas pindolol showed no such effect. Pindolol and bopindolol produced a significant increase in high-density lipoprotein cholesterol concentration, but did not produce an increase in serum creatine phosphokinase activity. Administration of these drugs in an acute regimen at 10 weeks of age, provoked changes in blood pressure and heart rate, reflecting partial agonist activities of varying degree, in agreement with the results obtained in chronic experiments with regard to serum lipoprotein concentrations. Pindolol (5 mg/kg) provoked a larger decrease of blood pressure without decrease of heart rate, while the decrease of blood pressure after bopindolol (5 and 15 mg/kg) was associated with a decrease of heart rate. It was concluded that bopindolol is a beta-adrenergic blocking agent with a mild partial agonist activity and devoid of adverse effects on lipid metabolism.