The rise in metabolic rate after intragastric feeding with fat and carbohydrate was enhanced in cold-acclimated (5°C) rats and diminished in warm-acclimated (30°C) rats compared to controls (24°C), but the response was largest in cold-acclimated animals intubated with fat. These acute effects of nutrients were almost completely abolished by β-adrenergic blockade with propranolol in all groups, while the parasympathetic antagonist atropine sulphate enhanced the responses in control rats, but had little effect in cold-acclimated animals. Feeding carbohydrate produced similar increases in interscapular brown adipose tissue (BAT) temperature in control and cold-acclimated rats, but fat caused a much greater rise in the latter group. The thermic effects of both nutrients were lower in genetically obese Zucker rats than in their lean littermates. Atropine slightly increased the thermic responses to fat and carbohydrate in the lean Zucker rats and caused marked potentiation in obese rats intubated with fat, but did not alter the effect of carbohydrate in the obese animals These data suggest that the size of the acute rise in metabolic rate after fat and carbohydrate is dependent on the thermogenic capacity of the animal. The response to fat was particularly large in cold-acclimated rats, where BAT activity is high, possibly due to a direct action of fat on the tissue. Genetically obese Zucker rats show defective thermogenesis which may be due to a combination of low sympathetic activity, high parasympathetic activity and insensitivity to substrate availability.