In lambs, the rapid increase in heat production after birth is due to initiation of nonshivering thermogenesis in brown adipose tissue (BAT). This occurs in conjunction with an increase in amount and activity of BAT specific uncoupling protein 1 (UCP1) (Clarke et al. 1997). UCP1 abundance and activity is low in fetal life but, within twelve hours of birth, there is an increase in the thermogenic activity of BAT and mRNA for UCP1. This ontogeny of UCP1 mRNA in BAT is very similar that of leptin, which is first detectable in the sheep fetus at 90 days gestation in fetal adipose tissue, its expression then increases up to term at 147 days (Yuen et al 1999). Leptin is a hormone which is thought to play a physiological role is in energy balance, it is primarily produced by white adipose tissue although there is evidence for its production in both brown adipose tissue and the placenta. Lambs born in the autumn are known to be smaller than those born in the spring (McCoard et al. 1997). It is not known if moderate changes in date of mating can influence birth weight or adipose tissue development. The present study aimed to determine whether date of mating could influence lamb birth weight, the abundance of BAT, UCP1, plasma leptin.