[3H]Dihydroergocryptine, a nonselective alpha adrenergic antagonist, the alpha-1 selective antagonist, [3H]prazosin and the alpha-2 selective antagonist, [3H]yohimbine, were used to study binding sites in rat renal membranes. To establish a correlation between binding and a biological function, the ability of alpha adrenergic agents to stimulate or inhibit vasoconstriction was quantified in vitro using an isolated perfused kidney preparation. Binding with each radioligand was rapid, saturable and specific. Moreover, the order of potencies of a variety of adrenergic agents, determined by competitive inhibition studies, suggested that the binding of each radioligand was to sites with alpha adrenergic specificity. The total number of binding sites in these rat renal membranes. determined with [3H]dihydroergocryptine (Bmax, 212 fmol/mg of protein; KD, 12.8 nM) was approximately equal to the sum of binding site concentrations determined with the alpha-1 and alpha-2 selective radioligands (Bmax, 57 and 170 fmol/mg of protein; KD, 0.85 and 20 nM, respectively). However, the alpha receptor mediating renal arteriolar vasoconstriction appeared to be of the alpha-1 subtype as there was a close correlation between the in vitro results and the binding data determined with [3H]prazosin (r = 0.93). In addition, in both the functional and [3H]prazosin binding studies, unlabeled prazosin ws 5 to 40-fold more potent than the nonselective antagonist, phentolamine, and 400- to 1500-fold more potent than the alpha-2 antagonist, yohimbine. These studies suggest that rat renal plasma membranes contain binding sites with both alpha-1 and alpha-2 adrenergic receptor specificity, in a ratio of approximately 1:3. Despite the preponderance of alpha-2 receptors, the alpha receptor mediating renal vasoconstriction appears to be of the alpha-1 type.