The present study investigated the contribution of adrenergic beta 1-receptor stimulation to the cardiovascular and renal effects of low-dose dopamine in eight normal, water-loaded humans. Metoprolol (100 mg) or placebo was administered orally at 08.00 h in a randomized, double-blind fashion on two different days. Renal clearance studies were performed during a 1 h baseline period, two 1 h periods with dopamine infusion (3 micrograms kg-1 min-1), and a 1 h recovery period. Cardiac output was measured by an ultrasonic Doppler method, and lithium clearance (CLLi) was used to estimate proximal tubular outflow. Baseline values of heart rate, systolic pressure and mean arterial pressure decreased with metoprolol compared with placebo, but cardiac output, effective renal plasma flow (ERPF) and glomerular filtration rate (GFR) were not significantly changed. Metoprolol significantly decreased baseline CLLi and sodium clearance (CLNa) by 19% (P <0.01) and 34% (P <0.01), respectively. Metoprolol blunted the dopamine-induced increases in heart rate and systolic pressure, but cardiac output increased to the same extent on both study days by 26% (placebo, P <0.05) and by 31% (metoprolol, P <0.01), respectively. With and without metoprolol, dopamine did not significantly change GFR, and the percentage increases in ERPF were similar on the two study days (40% (P <0.001) and 42% (P <0.001), respectively). Dopamine increased CLLi and CLNa by 31% (P <0.01) and 114% (P <0.01), respectively, with placebo, and by 36% (P <0.01) and 114% (P <0.01), respectively, with metoprolol. Values during infusion remained significantly lower with metoprolol compared with placebo.(ABSTRACT TRUNCATED AT 250 WORDS)