Twelve seriously malnourished Jamaican babies were investigated before and after treatment to discover whether they were unduly susceptible to heat in the malnourished state. On exposure to an environmental temperature of 38 °C at low humidity their mean rectal temperature increased at a rate of 0·75 °C/hour when they were malnourished, while after recovery it showed little change. During heat exposure total evaporative water loss in the recovered children was, on average, 44% greater than it was in the malnourished state, and in the malnourished children there was a significant inverse relation between total evaporative water loss and rise in rectal temperature. Measured sweat loss under a ventilated capsule after an intradermal diaphoretic injection in 7 of the children confirmed that sweating was impaired when they were malnourished. Rectal and skin temperatures at sweat onset were unchanged after recovery, indicating that central homoeostatic mechanisms were probably not disturbed. Peripheral vasodilatation in response to heat was unimpaired. Children with severe skin lesions had the most defective sweating, and malnutrition may damage the sweat glands as it does the hair follicles. It is very important to nurse malnourished children at the correct temperature; an environmental temperature of about 29 °C is satisfactory in most cases.