The thermic response to noradrenaline infusion was measured by indirect calorimetry in 40 newly referred obese subjects and 19 lean controls. A marked variation in response was found, more so in the obese than in the lean. In the lean group the lowest noradrenergic thermic response was 12.8 kJ but in the obese group 26 per cent had a thermic response less than this. When the thermic response to noradrenaline is expressed as a percentage of the resting metabolic rate the response was significantly (P less than 0.002) lower in the obese, 8.5 (s.d. 6.0) per cent compared with 13.8 (s.d. 5.1) per cent in the lean. The lowest value in the lean group was 7.8 and 42 per cent of the obese group had lower values. The thermic responses in the obese group did not correlate with glucose:insulin ratios measured both fasting and in response to oral glucose, or with the cumulative rises of either free fatty acids or glycerol during the noradrenaline infusion. Hence the majority of obese subjects have a 'normal' response to infused noradrenaline, but there is a subgroup who do have a blunted response. This may explain to some extent the conflicting reports in the literature regarding the presence of otherwise of a thermic defect in obesity.