Abstract. To characterize the type of alpha adrenergic receptor, the effects of specific alpha adrenergic agonists and antagonists on antidiuretic hormone ([Arg8]-vasopressin [AVP])-induced water absorption were evaluated in cortical collecting tubules isolated from the rabbit kidney and perfused in vitro. In the presence of AVP (100MU/ml), net fluid volume absorption (Jv, nanoliters per minute per millimeter) was 1.39±0.09 and osmotic water permeability coefficient (Pf, X i0-4 centimeters per second) was 150.2±15.0. The addition of 10-6 M phenylephrine (PE), an alpha adrenergic agonist, resulted in a significant decrease in J, and Pf to 0.72±0.11 (P < 0.005) and 69.9±10.9 (P < 0.005). The addition of 10-4 M prazosin (PZ), an alpha, adrenergic antagonist, did not cause any significant change in J, and Pf, which were 0.71±0.09 (P = NS vs. AVP + PE) and 67.8±9.5 (P = NS vs. AVP + PE), respectively. In a separate group of tubules, in the presence of AVP (100 ,U/ml) and PE (10-6 M), J. and Pf were 0.78±0.17 and 76.1±18.0, respectively. The addition of 10-6 M yohimbine (Y), an alpha2 adrenergic antagonist, resulted in a significant increase in J, to 1.46±0.14 (P < 0.01) and Pf to 157.5±22.3 (P < 0.005). Y (10-4 M) or PZ (10-4 M) alone did not significantly affect J, and Pf in the presence of AVP (100 ,uU/ml).The effect of the natural endogenous catecholamine norepinephrine (NE) on J, and Pf in the presence of AVP Dr. Krothapalli was a postdoctoral Fellow ofthe National Kidney Foundation during the course of these studies.Received for publication 14 June 1983 and in revisedform 22 November 1983. and propranolol (PR) was next examined. J. The effect of the alpha2 adrenergic agonist clonidine (CD) on J, and Pf was also examined. In the presence of AVP (10 ,uU/ml), Jv and Pf were 1.65±0.04 and 175.1 ± 13.1, respectively. The addition of CD (10-6 M) resulted in a significant decrease in Jv to 1.08±0.12 (P < 0.01) and Pf to 108.1±15.4 (P < 0.01). Increasing the concentration of CD to 10-4 M resulted in a further significant decrease in J, and Pf to 0.57±0.13 (P < 0.02 vs. CD 10-6 M) and 54.7±13.8 (P < 0.01 vs. CD 10-6 M), respectively. Similar results were obtained in the presence of AVP (100 uU/ml