Heat production rates of individual hornets were measured at 10, 15, and 20-degrees-C by means of direct calorimetry in order to investigate endothermic behaviour and capabilities to contribute to nest heat. Hornet workers showed heat production rates of 49 mW per g wet weight (w.w.) (SD = +/- 8 mW g-1) at 20-degrees-C; drones have lower heat production rates at the same temperature (36 mW g-1, SD = +/- 5 mW g-1). Nevertheless, the latter may contribute significant portions of heat to the nest at the end of season. The heat production of the whole colony was assessed at 1.8 W at the state of maximum biomass and at 1.3 W later, after the appearance of the reproductive forms, i.e. the drones and queens. The fact that drones and workers lowered their heat production rates with decreasing ambient temperatures in our experiments is discussed.