The dose-response effects of dopamine on kidney function were studied in rats. Th arterial blood pressure was increased only transiently, after continuous intravenous infusion of dopamine for 2 hr, while heart rate was elevated throughout. Drastic diuresis was usually observed during the early phase of infusion especially when the dose of dopamine was greater than 46 micrograms/min-kg BW. Natriuresis was also found to be dose-dependent but its duration was much more prolonged than diuresis. Both, the increase in glomerular filtration rate (GFR) and reduction in tubular salt reabsorption may be responsible for this elevation in the rate of sodium excretion. Dopamine, at low dose, insignificantly increased the GFR but at a higher dose (90 micrograms/min-kg BW), a 16% increment was observed. This elevation in GFR may not be due to a concomitant increase in the renal blood flow, since phenoxybenzamine, an alpha-adrenergic blocking agent, which would further reduce the renal vascular resistance, abolished the effects of dopamine on GFR, diuresis and natriuresis. It was suggested that the increase in GFR may be due to the differences in the responsiveness of the afferent and the efferent arterioles to dopamine, in the presence of normal alpha-adrenergic receptors. Dopaminergic receptors in the kidney of rat, if present, may not play a significant role in determining the renal function.