1. Five pregnant sheep, with indwelling catheters positioned for withdrawal of maternal right-atrial and foetal arterial blood, were exposed for 2 h to neutral and to cold (-10 and wool-clipped) environmental temperatures. Acute maternal cold exposure increased the concentrations of glucose, glycerol and non-esterified fatty acids in maternal plasma and increased the concentration of glucose in foetal plasma, but not glycerol or non-esterified fatty acids. The concentration of corticosteroids in maternal plasma increased. The concentration of corticosteroids in foetal plasma did not change but the concentration of insulin increased. 2. Thirteen pregnant sheep were housed at thermoneutrality and thirteen similar sheep, fed the same amount of food, were clipped and exposed to cold during the last 5-6 weeks of pregnancy. Chronic maternal cold exposure increased the birth weight of both single and twin lambs. 3. It is concluded that exposure of pregnant sheep to cold can alter the partition of some nutrients between mother and foetus in favour of the foetus, and it is suggested how this may be brought about.