This study examined the effect of ambient temperature and feeding on brown adipose tissue (BAT) function and thermoregulation in lambs born either vaginally at term or by Caesarean section close to term. Immediately after birth lambs were placed in a warm (30 degrees C) or cool (15 degrees C) ambient temperature and measurements of colonic temperature and heat production recorded for 6 h. Lambs were fed 50 ml of colostrum when 5 h old. The amount of uncoupling protein and level of guanosine 5'diphosphate (GDP) binding in BAT was higher in vaginally delivered lambs than in lambs delivered by Caesarean section. For each delivery group, GDP binding was greater in lambs maintained at 30 degrees C than in lambs maintained at 15 degrees C. O2 consumption, CO2 production and colonic temperature only increased after feeding in lambs born by Caesarean section and maintained at 30 degrees C, a response that was accompanied by a decreased incidence of shivering. Irrespective of delivery temperature, plasma thyroid hormone concentrations and noradrenaline content of BAT were lower in lambs born by Caesarean section than in those born vaginally. Plasma cortisol concentrations were higher in lambs delivered by Caesarean section, as was adrenaline content of BAT in these lambs maintained at 30 degrees C. It is concluded that the thermoregulatory response to feeding in terms of changes in both recruitment of shivering and colonic temperature were observed only in lambs delivered by Caesarean section.