The postnatal development of renal function was compared in infants with a gestational age of 25-30 weeks, mean 27.8 weeks (GA 28), and in infants with a gestational age of 31-34 weeks, mean 32.5 weeks (GA 32). The infants were comparable with regard to postnatal course, fluid, caloric and salt intake. Observations were made during the 1st, 2nd and 4th-7th (mean 5th) postnatal weeks. From the 1st to the 5th postnatal week the creatinine clearance (CCr ml/min/1.73 m2), increased from 11 to 20 in GA 28 and from 15 to 30 in GA 32. At 2 weeks of age CCr was significantly lower in GA 28 than in GA 32. During the first week of life diuresis was lower in GA 28 than in GA 32 but thereafter was the same in both groups. We interpret this as a sign of dehydration in GA 28. Serum arginine vasopressin (S-AVP) concentrations were high in both groups at all ages. Mean urine osmolality was low (less than 300) regardless of postnatal age and S-AVP. Urinary sodium excretion was high at 1 week of age in both groups and decreased with increasing postnatal age. Na excretion was slightly higher in GA 28 than in GA 32 at 1 but not at 2 and 5 weeks. UK/UNa was below 1 in both groups during the first week of life and increased with postnatal age. Urinary aldosterone excretion was high in both GA 28 and GA 32 at all ages. Serum sodium levels were lower in GA 28 than in GA 32 at all ages.(ABSTRACT TRUNCATED AT 250 WORDS)