1. Pigs were weaned at 8--10 dyas of age and littermate pairs were raised in individual cages at ambient temperatures of 25 degrees C or 35 degrees C to 8 weeks of age. 2. Pigs reared at the higher temperature had longer extremities and less hair than controls even though body weights were similar. 3. On exposure to an ambient temperature of 45 degrees C, thce as quickly as their littermates. 4. When the scrotum was heated locally to 42 degrees C animals reared in the warmer environment began to pant at a lower ambient temperature than the controls. 5. Heating the hypothalamus by means of an implanted thermode at an ambient temperature of 35 degrees C was accompanied by an increase in respiratory frequency in pigs reared at 25 degrees C. The pigs reared at 35 degrees C already had a slightly elevated rate of breathing and heating the hypothalamus caused no change. 6. The rate of blood flow in the tail was higher in pigs reared at 25 degrees C than those kept at 35 degrees C when measured at ambient temperatures between 20 degrees C and 35 degrees C. When the hypothalamus was cooled the slope of the line relating blood flow to the temperature of the thermode was similar in both groups. 7. The critical temperature as determined from measurements of oxygen consumption was higher in the pigs reared at 35 degrees C than in controls. But the effect of cooling the hypothalamus on metabolism was similar in both groups. 8. There was no difference between the two groups in the rate of cutaneous water loss.