The present study was designed to investigate the role of noradrenaline (NA), dopamine (DA), and serotonin (5-HT) in the adaptation of premature infants to alterations of sodium balance. Urinary excretion of NA, DA, and 5-HT was measured spectrofluorimetrically in a group of low birth weight premature infants with (group S) and without (group NS) NaCl supplementation. Group NS consisted of 10 infants with a birth weight of 1200-1750 g (mean, 1493 g) and gestational age of 28-31 wk (mean, 30.1 wk). Group S included 10 infants with mean birth weight of 1414 g (range, 1150-1600 g) and mean gestational age of 30.5 wk (range, 27-32 wk). Measurements were made on the 7th day and weekly thereafter until the 5th wk of life. NaCl supplementation was given in a dose of 3-5 and 1.5-2.5 mEq/kg/day for 8-21 and 22-35 days, respectively. In group NS, mean urinary excretion of NA and DA increased from 8.6 +/- 1.5 and 15.8 +/- 2.4 micrograms/day to maximum values of 21.4 +/- 5.5 (p less than 0.05) and 33.4 +/- 6.0 micrograms/day (p less than 0.01) in weeks 2-3, respectively. 5-HT excretion averaged about 60 micrograms/day and showed no consistent change during the course of the study. NaCl supplementation prevented the rise of NA and DA excretion above the initial baseline values. The postnatal course of 5-HT excretion, however, remained unaffected by NaCl supplementation. Urinary excretion of NA in weeks 2-3 (p less than 0.05) and DA in weeks 2-4 (p less than 0.05) were significantly lower in group NS.