Renal hemodynamics and functions were assessed in four groups of anaesthetized newborn rabbits receiving a single intravenous dose of methylxanthines, i.e.: aminophylline: 3 mg/kg (A1) or 6 mg/kg (A2); sodium benzoate caffeine: 5 mg/kg (C1) or 10 mg/kg (C2). Each animal acted as its own control. The mean PAH extraction ratio was not modified by the methylxanthines. Renal blood flow and glomerular filtration rate were determined by clearances of para-aminohippuric acid and inulin, respectively. No changes in renal hemodynamics or renal functions were observed in group C1. In group C2, renal blood flow and filtration fraction did not vary significantly but renal vascular resistance showed a delayed increase. Systemic infusion of the two aminophylline regimens induced a delayed increase in renal vascular resistance with a concomitant fall in renal blood flow and an increase of filtration fraction in group A2. Glomerular filtration rate was either reduced (3 mg/kg aminophylline) or increased (6 mg/kg aminophylline and 10 mg/kg caffeine). Moreover, diuresis increased and tubular water reabsorption declined in groups A1, A2, and C2. High dose caffeine enhanced sodium fractional excretion. The acute renal effects of methylxanthines appeared dose- and time-related in immature animals and caffeine proved safer than aminophylline at doses used in human neonates.