Renal function was studied in 15 newborn New Zealand rabbits administered either 0.6 mg/kg enprofylline intravenously. Each animal acted as its own control. Glomerular filtration rate (GFR) and renal blood flow (RBF) were assessed by the clearances of inulin and para-aminohippuric acid, respectively. Enprofylline, a xanthine with low adenosine antagonistic properties, did not modify urine flow rate, GFR, RBF, renal vascular resistance, filtration fraction, sodium and potassium urinary excretion, whereas administration of theophylline, a potent adenosine antagonist, was associated with a significant increase in diuresis, renal vascular resistance and filtration fraction. The differences observed in the renal effects of theophylline and enprofylline strongly support the view that 1) the renal actions of micromolar concentrations of theophylline are mediated by an antagonism with endogenous renal adenosine; 2) renal adenosine could play a physiological role in the regulation of renal hemodynamics.