The study investigated the effect of age, feed restriction and photoperiod during rearing upon mature liveweight, body composition and sexual maturity in ducks. The literature on wild Mallard is extensive but there is very little published evidence relating to d~estic duck. Experiments were conducted with Pekin duck selected for meat production. Unrestricted growth to maturity was shown to produce uncoordinated development of ovarian follicles which caused internal OVUlation and reduced rate of lay. Restricting growth by limiting feed intake during rearing delayed physiological development and sexual maturity and improved synchronization between growth and sexual development which improved breeding performance. Severe restriction of feed intake up to 22 weeks reduced mature IIveweight and delayed sexual maturity. There was a linear relationship between liveweight of 1;reatment groups at 20 weeks of age and mean age at sexual maturity. However, it was found that ducks must attain at .Ieast 65 percent of the liveweight of ad-libitum fed controls before they could achieve sexual maturity. Opportunities for altering body composition by varying nutrient intake during rearing were shown to be very limited. There was a fixed relationship between body composition and bodyweight. Changes in natural or artificial day length during rearing were shown to affect age at sexual maturity but the response was affected by restricted feeding. Delaying sexual maturity by restricted feeding and controlling daylength during rearing both increase subseQuent rate of lay, egg production and egg weight but there "as no improvement in breeding performance measured to a fixed age, as a result of delaying sexual maturity beyond 26 weeks of age. 1 2 Equations are given for responses 01 body'weight, sexual maturity, egg number and egg size to feed and light treatments. These could be used to build a predictive model for practical use.