The reported effects of renal alpha 2-adrenoceptor blockade on sodium and water excretion have been inconsistent. We therefore studied the effect of an intrarenal infusion of an alpha 2-adrenoceptor antagonist in rats undergoing two distinct levels of diuresis and natriuresis. Renal excretion of sodium and water was studied in anesthetized rats that had been unilaterally nephrectomized (right kidney) 10 days prior to the experimental day. In the presence of the lower rate of saline infusion an intrarenal infusion of the alpha 2-adrenoceptor antagonist, yohimbine, (25.6 nmol/kg per min) resulted in no change in urine volume or sodium and potassium excretion. In the presence of the modest diuresis, due to the higher level of saline infusion, intrarenal yohimbine resulted in a decrease in urine volume, sodium excretion and free water clearance. These effects of yohimbine were not found in adrenalectomized rats. The ability to demonstrate an effect of renal alpha 2-adrenoceptor blockade was dependent on the baseline level of sodium and water excretion. These results suggest that renal alpha 2-adrenoceptors may mediate the inhibition of the renal action of vasopressin by adrenal catecholamines.