Renal functions were assessed during two 12-hour periods, before and after the administration of a single oral dose of caffeine (15 mg/kg) in 13 preterm infants. Each infant acted as his own control. Heart rate, mean blood pressure, hematocrit, blood pH, arterial PO2 and PCO2 remained stable throughout the two successive periods. Mean protein serum level slightly increased from period I to period II. Urine flow rate (+63 +/- 57%), water output/input ratio (+49 +/- 54%) and creatinine clearance (+79 +/- 102%) increased significantly after the administration of caffeine.