In order to study the mode of inheritance of traits associated with the onset of sexual maturity, data from stocks differing greatly in genetic background were analyzed when populations achieved 50% lay. Chickens used in this experiment consisted of three parental lines and five crosses. Parental lines were White Plymouth Rocks bidirectionally selected for high (HH) and low (LL) 56-day body weight, and a closed population of Jersey Black Giants (JJ). Crosses were produced from reciprocal matings of the White Rock lines (HL and LH), their F2 (HL X HL), and Jersey males mated to weight-line females (JH, JL). When 50% of the pullets within a population had laid, various traits were measured for each individual in that population. Retrospective analyses showed that sexually mature pullets were heavier at 91 and 126 days of age than those which had not commenced lay. Body weight dimorphism between layers and nonlayers was disproportionately greater in LL and F2 pullets than in those of other populations, resulting in a population by physiological stage interaction. Fat depots were heavier and percentages of fat were higher in laying than nonlaying pullets. Evidence was obtained suggesting that an absolute quantity of breast tissue was necessary before lean tissue percentage became meaningful as a threshold trait for the onset of egg production. Modes of inheritance for body weight and compositional characteristics were different in sexually immature and mature pullets, suggesting that muscle functioned as a nutrient reserve in laying pullets and a nutrient store in those that had not reached this physiological stage.