The effect of prenatally administered glucocorticoids on tubular reabsorption of beta 2-microglobulin and elimination half-life gentamicin have been investigated in newborn infants. beta 2-Microglobulin:creatinine ratio was significantly higher in preterm (4.43 +/- 0.88) than in full-term infants (0.89 +/- 0.42), but did not differ between infants exposed to betamethasone in utero and those who were not. Gentamicin pharmacokinetics did not differ between preterm infants who had or had not been exposed to betamethasone. It is therefore concluded that whilst prenatally administered glucocorticoids influence both pulmonary and hepatic maturation, they do not alter either renal tubular reabsorption of beta 2-microglobulin or glomerular filtration rate as estimated by gentamicin half-life.