The purposes of these experiments were to determine the renal effects of 2-chloroadenosine and to determine whether the effects could be antagonized by aminophylline. Three groups of rats were anesthetized with Na pentobarbital and given 0, 9 or 18 nM/min of 2-chloroadenosine i.v. Dose-dependent reductions in mean arterial blood pressure, filtration fraction, inulin clearance, urine flow and K excretory rate resulted. Also, Na excretory rate and fractional Na excretion were reduced and plasma K was increased in both groups receiving 2-chloroadenosine, but these changes were not greater in animals receiving the higher dose. Intravenous aminophylline antagonized the 2-chloroadenosine-induced effects on plasma K, mean arterial blood pressure, inulin clearance, urine flow and Na and K excretory rates. 2-Chloroadenosine is an adenosine receptor agonist and aminophylline an adenosine receptor antagonist in many types of cells. These observations are consistent with the existence of adenosine receptors on rat nephrons, possibly on tubular as well as vascular components.