Heat production of isolated brown-fat cells by addition of noradrenaline adrenaline and glucagon was measured in warm-acclimated control, coldacclimated and heat-acclimated rats by use of a twin-type conduction microcalorimeter. Noradrenaline and glucagon induced maximum heat production per 10⁶ cells in dose of 1 μg/ml. Heat produced by maximum thermogenic response to glucagon was twice as much as that to noradrenaline. Thermogenic response to noradrenaline was markedly increased in cold-acclimated brown adipocytes, while it was reduced in heatacclimated ones. Thermogenic response to glucagon was significantly reduced in heat-acclimated brown adipocytes, while it was not affected in cold-acclimated brown adipocytes.