The pharmacodynamic effect of propranolol (80 mg b.i.d.) on the renal and systemic circulation was studied after 1 and 8 months of treatment in 13 patients with essential hypertension, using non-invasive radioisotope techniques. Effective renal plasma flow (ERPF) fell from (mean +/- S.E.M.) 244 +/- 18 to 208 +/- 14 after 1 month and to 187 +/- 13 ml/min.m2 after 8 months of treatment. Concomitantly cardiac index (CI) fell from 3.24 +/- 0.15 to 2.62 +/- 0.11 and 2.75 +/- 0.10 l/min.m2, respectively. The coefficient of correlation between the decreases in ERPF and CI was 0.49. Mean arterial blood pressure decreased from 138 +/- 5 to 118 +/- 5 and 116 +/- 4 mmHg, respectively. Left ventricular work was reduced by 30.2 and 27%, while peripheral resistance was unchanged. Total plasma volume was increased from 19.3 +/- 0.5 to 20.3 +/- 0.6 ml/cm after 1 month, but was within the same range as the control values after 8 months of treatment. Pulmonary plasma volume was unchanged, indicating that there was no pooling of plasma in the pulmonary circulation. The interventricular circulation time was increased from 6.9 +/- 0.4 s to 8.4 +/- 0.3 s and varied with the change in heart rate. It is concluded that the fall in ERPF might be explained by reduced cardiac output in addition to interference with the hemodynamic autoregulation in the kidney.