In rats maintained on a scheduled feeding plan, the hypertrophy of brown adipose tissue (BAT) observed after a low-protein/high-carbohydrate single test meal was accompained by significant changes in the percentage of all major constituents of the tissue. There was a fall in the percentage of water (P < 0.01), a rise in the percentage of fat (P < 0.05), and a rise in the percentage of glycogen (P < 0.001). The largest absolute changes following a meal were in the fat content, which almost doubled, and in the glycogen content, which exhibited about a four-fold increase. Deposition of fat in the BAT following the test meal was accompanied by a three-fold increase in the rate of fatty acid synthesis (P < 0.05). The in vitro respiration rate of BAT was usually significantly increased in the meal-fed rats, but the effect of replacing the protein content of the test meal with starch was not clear. A lower protein, higher starch diet (9% of calories from protein, 72% from starch) resulted in a trend for a larger thermic effect than a higher protein, lower starch diet (27% of calories from protein, 54% from starch).