The thermal components of the environment, through their effects on the animals' heat exchange, influence growth, metabolism and voluntary food intake. The environmental components considered include air temperature, either constant or fluctuating, air movement, relative humidity, group size, stocking density and atmospheric concentrations of various gases and dusts. For the growing pig, it has been calculated that each 1°C change in temperature is associated with a 0.65 MJ metabolizable energy (ME) per day change in energy intake; for the other environmental components the variation was between 0.36 and 0.65 MJ ME per day. From the relation between environmental temperatures, food intake and growth, it was shown that for young pigs the optimum temperature range was between 20 and 25°C; for older animals it was between 10 and 20°C. Knowledge of the extent to which food intake changes with the environmental circumstances, both cold and hot, allows feeding and managemental strategies to be developed to ensure optimum growth and efficiency of food utilization.