Egg and meat type hens have been selected differently over the past decades. While breeding programs for laying hens have been focused in improving egg production, broiler breeder hens have been intensively selected for growth and feed efficiency. Recent evidence suggests that both laying hens and broiler breeders hens do not exclusively rely on the photostimulation to commence sexual maturation. Since limited emphasis was put on the possible metabolic similarities and little is known about the physiological conditions regarding the body compositions of the laying and broiler breeder pullets at the time of sexual maturation, a series of studies will be carried out aiming to understand better the physiological conditions underlying the body composition of laying and broiler breeder hens.