1. The effect of a single dose of lithium on renal function before and during intravenous infusion of dopamine (3 micrograms min-1 kg-1) was investigated in 12 healthy males. In a double-blind and randomized design, 450mg or 600mg of lithium carbonate or placebo was administered orally at 22.00 hours on three different occasions. After an overnight fast, the subjects were water-loaded and clearance studies were started at 09.00 hours with a 1h baseline period and three 1h periods during dopamine infusion. 2. Baseline sodium clearance with placebo was 0.65 +/- 0.35 ml/min, but with lithium it increased to 1.25 +/- 0.44 (P <0.001) and 1.17 +/- 0.46 ml/min (P <0.01) after 450 and 600mg, respectively. Urine flow rates were unchanged compared with placebo. Lithium did not significantly affect glomerular filtration rate, but both doses slightly increased effective renal plasma flow by 7% (P <0.05) and 10% (P <0.01), respectively. 3. The maximal natriuretic and diuretic effects of dopamine were not reduced by lithium, but the percentage increases in sodium clearance were significantly diminished after 450mg (P <0.01) and 600mg (P <0.001) of lithium. Lithium had no effect on dopamine-induced changes in effective renal plasma flow, glomerular filtration rate or osmolal clearance. Neither lithium nor dopamine influenced plasma concentrations of renin, aldosterone or atrial natriuretic peptide. 4. In conclusion, single test doses of lithium, as normally used in lithium clearance studies, increase baseline values of sodium clearance and effective renal plasma flow.(ABSTRACT TRUNCATED AT 250 WORDS)