The effects of the selective alpha-1 adrenoceptor agonist, cirazoline, and the selective alpha-2 adrenoceptor agonist, B-HT 933, were assessed on renal hemodynamics and on water and solute excretion in conscious, chronically instrumented rats. Infusion (i.v.) of equipressor doses of cirazoline and B-HT 933, 0.04 and 4 mg/kg/hr, respectively, decreased renal plasma flow without changing glomerular filtration rate. Cirazoline infusion did not affect urinary excretion of water, electrolytes or total solutes. In marked contrast, B-HT 933 increased urine flow and sodium excretion significantly (P less than .01) but did not significantly alter potassium and urea excretion. Urine osmolality decreased to hyposmotic levels (from 613 +/- 86 to 172 +/- 8 mOsmol/kg of H2O) during the infusion of B-HT 933, suggesting a possible interaction between the alpha-2 adrenoceptor agonist and the vasopressin system. This diuretic action of the selective alpha-2 adrenoceptor agonist was also observed after the i.v. infusion of a subpressor dose (0.4 mg/kg/hr) of B-HT 933. In rats treated with the ganglionic blocker, hexamethonium (10 mg/kg i.v.), the B-HT 933-induced diuresis was not affected, confirming an action in the periphery, possibly at the level of the kidney. These results suggest that stimulation of renal alpha-2 adrenoceptors in conscious, euvolumic rats modulates the reabsorption of water and sodium at the site of the renal nephron.