Insensible water loss (IWL) was measured by an Electronic Potter Baby Scale in 170 healthy preterm infants, with birthweights ranging from 800 to 2,000 gm, who were appropriate for gestational age. IWL was found to increase with decreasing birth weight, i.e., from a mean of 0.7 ml/kg/hr in infants weighing 1,751 to 2,000 gm to 2.67 ml/kg/hr in infants weighing < 1,000 gm. This correlation between IWL and birthweight was high (r = 0.91; P = < 0.001). In infants < 1,500 gm, IWL decreased with postnatal age while in infants > 1,500 gm, IWL increased in the second week and was stable in the third and fourth weeks. Exposure to non-ionizing radiant energy from three radiant heat warmers was found to increase IWL by 50% to 190% depending on the maturity of the infant and the type of warmers. Phototherapy was associated with a twofold to threefold increase in IWL, which can be minimized by careful temperature control of the infant. The magnitude of increase in IWL in response to exposure to non-ionizing radiant energy, both from radiant heat warmers and phototherapy was greater in infants with birthweight > 1,500 gm than in infants < 1,500 gm.