Circadian variation in the heat production rate of young broiler chickens and laying broiler hens was studied by indirect calorimetry. 2. Variation in heat production rate was synchronised by the light cycle both in the 23-h light:1-h dark regime of the growing chickens and the 14-h light:10-h dark regime of the laying hens. 3. The relative amplitude of the decrease during the dark phase was about twice as great during the 10 h of darkness afforded the adult hens as it was during the 1-h period of darkness experienced by the growing birds.
