To investigate the changes in body composition corresponding to postnatal weight loss and regaining of birth weight, total body water (TBW) and extracellular water (ECW) were measured at birth, on the day of maximum weight loss and on regaining of birth weight in 23 preterm appropriate for gestational age (AGA) infants (mean +/- SD birth weight 1902 +/- 242 g, gestational age 236 +/- 7 days). Intracellular water (ICW) was determined by the difference between TBW and ECW and body solids by the difference between TBW and body weight. Almost 90 per cent of early postnatal weight loss of 132 +/- 38 g (6.9% of birth weight) was because of loss of body water (117 +/- 30 ml; 7.9% of TBW at birth). ECW loss (mean +/- SD 106 +/- 35 ml) accounted for 90 per cent of the TBW loss. Of the subsequent weight gain (134 +/- 40 g) till regaining of birth weight, 48 per cent (64 +/- 28 ml) was TBW and 52 per cent (70 +/- 13 g) body solids. The major gain in body water was in ICW (47 +/- 21 ml). A gradual decrease in TBW and ECW, and a gain in ICW and body solids per kg body weight was observed throughout the study period. These findings favour the concept that in preterm (31-36 wk) infants (i) postnatal weight loss is primarily a reflection of ECW loss and subsequent weight gain is because of cellular growth, (ii) postnatal loss of ECW continues even when weight gain and accumulation of body solids has started.