Optimal temperature ranges of fattening pigs at different weights were assessed by examining the behavioural and adrenocortical responses of pigs to a range of ambient temperatures. Cortisol concentration was analyzed by taking saliva samples from the pigs. Behaviours measured included lying behaviour. Depending on ambient temperature, pigs chose different areas for resting. When temperature increased, pigs would lie in the dung area away from penmates more often. Heavier pigs lied without contacting their penmates more often than their lighter counterparts. Lying without penmate contact occurred 5 to 7 °C lower than lying in the dung area. As temperature decreased, huddling behaviour increased. As pig weight increased, huddling occurred more at lower temperatures. For pigs over 85 kg body weight, there was a significant increase in cortisol levels when ambient temperature increased. The results of the present study indicated that, for pigs weighing 25 to 35 kg and housed on partially slatted floors, temperature range within the thermal tolerance of pigs was 19 to 21 °C. At 50 to 70 kg body weight, the range was 10 to 17 °C. For pigs over a body weight of 85 kg, temperature range within their thermal tolerance was 5 to 17 °C.