The heat loss for individual growing pigs, whose initial body-weight was 25-40 kg, was measured continuously for periods of 14 or 21 d when they were maintained at an environmental temperature of either 20 or 30 degrees. For each temperature the feeding level before and after a period of fasting was held at either 39 or 45g/kg body-weight per d. Four pigs were given each treatment; food was withheld from three animals for a 4 d period in each experiment, and the fourth pig (control) was fed for the whole experimental period. 2. After the withdrawal of food, the heat loss decreased rapidly and minimum values were recorded during the third and fourth days of fasting. Fasting heat loss was higher at 20 degrees than at 30 degrees and was not significantly dependent on the preceeding level of feeding. The mean values obtained at 20 and 30 degrees were 457 and 380 KJ/kg 0.75 per d respectively. 3. The heat loss during the periods of feeding was significantly higher (P less than 0.05) at 30 degrees than at 20 degrees, and was also higher at the higher level of feeding. 4. The apparent efficiency of utilization of metabolizable energy for maintenance was calculated as 0.80. 5. It was concluded that the critical temperature for a fasting pig weighing 25-40 kg is approximately 25 degrees.
