Newborns usually lose 5-10% of their initial weight during the first 4 days of life. Our aim, using bioelectrical impedance (BI), was to study the body composition of healthy term newborns and the nature of the changes which accompany this physiological weight loss. Forty-three healthy term newborns, all with adequate weight for their gestational age, were studied during the first 3 days of life. Weight and BI were taken on the 1st, 2nd, and 3rd days of life, always at the same time of the day. Total body water (TBW), percentage of total body mass hydration (%TBW), and amount of body solids were calculated. Average weight at birth was 3,297+/-381 g, length 50.04+/-1.75 cm, and gestational age 39.9+/-0.84 weeks. Weight, TBW, and body solids decreased progressively during the first 3 days of life (p<0.000). By day 3, weight loss represented 5.67+/-1.98% of birth weight, but %TBW increased slightly (1.72%; p<0.000). These results suggest that, during physiological weight loss, body composition modifications are produced in the term newborn by a decrease in TBW and loss of body solids. The level of body hydration increases slightly, since the loss of body solids is greater than the loss of TBW. The use of BI can help us in the study of body composition and the short- and/or long-term changes produced therein, in the newborn.