Insensible water loss (IWL) was measured in six premature infants, between 4 and 21 days of age, by continuous weight monitoring on an electronic balance inside an incubator. Multiple measurements of IWL were made during the sequential infusion of 10% dextrose in 0.225% NaCl, 10% dextrose-amino acid solution, or 10% dextrose-amino acid and a commercial intravenous fat emulsion. Each solution was administered for three hours by constant infusion through a scalp vein needle. The order of the infusion was random and a 30- to 60-minute infusion with 5% dextrose water was given between each solution. During the infusion of 10% dextrose in 0.225% NaCl and 10% dextrose + amino acid solution, IWL was 1.0 +/- 0.8 gm/kg/hr and 1.1 +/- 0.8 gm/kg/hr, respectively. In contrast, IWL increased significantly to 1.6 +/- 0.7 gm/kg/hr when additional calories were given using the 10% dextrose-amino acid with the intravenous fat emulsion (P less than .005). There was a positive correlation between calorie intake and IWL. These data suggest that parenteral nutrition solutions with intravenous fat emulsion are rapidly metabolized and the increase in IWL is probably secondary to an increase in thermogenesis.