Creatinine clearance and renal sodium excretion were measured consecutively in three groups of 12 premature infants (gestational age less than or equal to 35 weeks) whose mothers had received either steroids or aminophylline, or steroids and aminophylline before delivery. We found no significant differences for plasma and urine creatinine and its clearance in the groups considered. The steroid group presented urine osmolality and urine/plasma osmolality ratio significantly higher than among the other groups. Furthermore, urine potassium excretion increased, and urine sodium and sodium fractional excretion were reduced. Aminophylline exposure did not interfere with the hydrosaline equilibrium nor with renal function of the preterms at birth. Our results reconfirm that corticosteroid hormones play an important part in the fetal renal maturation process, inducing a precocious maturation of the tubular Na(+)-K+ ATPase enzymatic system, that is substantially unmodified by aminophylline exposure. However, due to the prolonged half-life of aminophylline in prematures, it seems reasonable to verify the coupling of tubular and glomerular functions also in the following days of life.