Renal function was measured during the first 4 postnatal days in 9 preterm neonates (gestational age 26.2 to 31 wk) exposed to indomethacin during the last 2 days of pregnancy (group I). The data were compared to those obtained from nine control neonates (gestational age 28 to 34.5 wk) (group II). Five of the nine neonates in group I were markedly edematous at birth, none of group II were edematous. Urine production in group I was low (32.2 +/- 16.8 ml/kg.day on day 1 increasing to 68.6 +/- 21.4 ml/kg.day on day 4) and differed significantly from group II [75.2 +/- 26.8 ml/kg.day on day 1 increasing to 84.8 +/- 20.9 ml/kg.day on day 4 (p less than 0.001)]. Fluid intake was adapted to urine production when necessary. A continuous inulin infusion was started directly after admission and continued for 5 days. Renal function was evaluated for 3 consecutive days after at least 48 h of insulin infusion. The values of the inulin clearance, serum creatinine, urine osmolarity, osmolar clearance, and free water clearance were stable in both groups during the study period. Inulin clearance was lower in group I than in group II (p less than 0.001), whereas serum creatinine was higher in group I than in group II (p less than 0.0001). Urine osmolarity was higher in group I (p less than 0.01), whereas osmolar clearance and free water clearance were lower in group I (p less than 0.02, respectively, p less than 0.01). There was no difference in fractional sodium excretion between the groups.(ABSTRACT TRUNCATED AT 250 WORDS)