Creatinine clearance (Ccr) and renal sodium (Na+) excretion were measured in 10 premature infants (gestational age less than 34 weeks) whose mothers had received dexamethasone before delivery (group D) and in 11 whose mothers were not so treated (control, group C). Babies were studied twice: on days 2-5 (study 1. all infants) and days 6-10 (study 2, six infants in each group). In study 1, absolute and fractional Na+ excretion were significantly lower (P less than 0.01) and urinary K+:Na+ ratio significantly higher (P less than 0.025) in group D than in group C, while Cr did not differ between groups. In study 2, Ccr in group D had increased compared both with values obtained in the same babies in study 1 (P less than 0.05) and with group C babies in study 2 (P less than 0.05), but significant differences between groups in urinary Na+ excretion and urinary K+:Na+ ratio were no longer found. We conclude that exogenous glucocorticoids accelerate maturation of renal function in immature human infants, probably by inducing tubular Na+. K(+)-ATPase activity. Our findings support the view that endogenous glucocorticoid hormones may play an important part in the normal maturation process.