During the control period in this study, small premature infants, 1500 g in birth weight, had insensible water loss (IWL) of 2.28 g/kg/hr (equivalent to 54 ml/kg/day), which is considerably higher than in full term infants. Under phototherapy, there was a significant increase in IWL (3.14 g/kg/hr) associated with increase of skin and rectal temperature. The heart rate and respiratory rates were comparable.Full term infants in stress situations such as photo-therapy can compensate for water losses by increasing oral intake. In small premature infants, reliance must be placed upon increased amounts of parenteral fluids when under phototherapy.