In 5 healthy young males who received 40 mg oxprenolol given orally in a single dose, the hemodynamic effects, plasma renin activity and blood level of oxprenolol were investigated. One hour after administration of the drug, cardiac output and heart rate decreased by 10-20 %, remained subnormal for 4 h, but returned to control levels after 6 h. Decreases in the cardiac index were directly related to decreases in the heart rate with the stroke volume only minimally affected. Mean blood pressure remained unchanged. The plasma renin activity was significantly decreased from a mean of 0.77 ng/ml/h before the administration to 0.38 ng/ml/h after 1 h and returned to the control values after 4-6 h, suggesting the β-adrenergic system to be one of the maintenance mechanisms of renin secretion. The peak concentration of oxprenolol was observed after 1 h and it decreased rapidly after 2 h. The high blood levels coincided with the hemodynamic changes and renin suppression, but these effects could still be observed, when blood oxprenolol levels had returned to rather low levels.