The hemodynamic effects provoked by 15 days treatment with atenolol 100 mg/day in a single oral dose were studied in labile hypertensive patients. Radiocardiography with 131IRHISA was employed in order to calculate: Cardiac output and index (CO, CI); stroke volume and index (SV, SI), total blood volume and index (TBV, TBVI) and total peripheral resistance (TPR). Atenolol caused a significant reduction in arterial pressure, heart rate, CO and CI (p less than 0.001). The drug also reduced TBV (11%) and TBVI (16%) from controls: however, these changes were not significant. A direct relationship was found between CO with heart rate (r = 0.79, p less than 0.01), and between CO with TBV (r = 0.82, p less than 0.01); also an inverse relationship was found between TBV with TPR (r = 0.874, p less than 0.01). These relationships lost significance during treatment with the drug. The lowering of cardiac output contributes to the anti-hypertensive effect of atenolol, but although the reduction in blood volume was not significant it is possible that this also may be a factor.