1. The thermogenic response and changes in plasma substrates and hormones were tested after a liquid meal in lean, obese and formerly obese women. 2. Subjects with a family history of obesity tested either while obese or after slimming to a normal weight had a thermogenic response, which was only half that of the lean group. 3. The immediate response in plasma glucose and insulin was greater in the lean subjects, but the sustained changes in circulating substrates did not differ in the three groups. Thyroidal hormone concentrations did not alter postprandially: venous noradrenaline levels rose in the obese groups only. 4. The differences in postprandial thermogenesis at rest would reduce the energy requirements of subjects with familial obesity, but they still had a metabolic rate estimated to be nearly 1MJ (240 kcal)/day in excess of the lean group so additional thermogenic defects must exist for familial obesity to be explained solely on a metabolic basis.