To establish the effects of alpha-adrenoceptor blockade on dopamine-induced changes in renal hemodynamics and sodium excretion, dopamine dose-response curves were performed without and with pretreatment with the selective postsynaptic alpha 1-adrenoceptor antagonist prazosin in normal volunteers and in patients with renal disease and moderately impaired renal function. Prazosin (1 mg p.o. every 4 h) in 7 volunteers did not significantly affect baseline values but impaired the response of effective renal plasma flow (ERPF) and filtration fraction (FF) to infusions of dopamine in doses ranging from 0.5 to 8 micrograms/kg per minute and completely abolished the dopamine-induced increase in sodium excretion. In 7 patients with renal disease and a glomerular filtration rate (GFR) ranging from 38-85 ml/min pretreatment with prazosin did not affect baseline ERPF, GFR or FF or their response to dopamine infusion, but sodium excretion and its response to dopamine infusion were reduced (fractional excretion of sodium at baseline 1.78 without and 0.89 with prazosin pretreatment). We conclude that alpha 1-adrenoceptor blockade with prazosin abolishes the effects of exogenous dopamine on sodium excretion in normal man. Prazosin also impairs the renal vasodilatory action of dopamine. However, the effect on sodium excretion is not directly related to inhibition of dopamine-induced renal vasodilation since in patients with renal disease prazosin also markedly reduces sodium excretion but does not influence the renal hemodynamic effects of dopamine.