The heat losses and energy balances of thirty-eight individually housed pigs (initial body-weights 21--38 kg) were measured continuously for periods of 14 d when they were maintained at environmental temperatures of 10, 15, 20, 25 or 30 degrees. At each temperature four levels of feeding were given approximating to once, twice and three times the maintenance energy intake and the ad lib. level. The minimal maintenance energy requirement (M) was calculated to be 440 kJ metabolizable energy (ME)/kg0.75 per d at 25 degrees. 2. ME intake at the ad lib. level decreased from 1965 kJ/kg0.75 per d at 10 degrees to 1202 at 30 degrees. 3. Heat loss calculated from multiple regression analysis decreased to minimum levels between 20 and 25 degrees; 30 degrees was within the hyperthermic zone at each plane of nutrition. 4. The partition of heat loss into its sensible and evaporative components showed that evaporation increased from 25% at 10 degrees to 78% at 30 degrees. 5. Critical temperature was dependent upon food intake and decreased from 23.1 degrees at M to 20.7 degrees at 2M, 18.0 degrees at 3M and 16.7 degrees at 4M. 6. The extra food required to meet extra thermoregulatory heat production per 1 degrees below the effective critical temperature was 0.65 g/kg body-weight per d.
