1. The heart rates (fH) of 12-day-old embryos (young), 16- and 18-day-old embryos (late) and of 20-day-old embryos (externally pipped eggs, EP) were measured noninvasively at a temperature of 38 degrees C and after a 5 hr exposure to an ambient temperature (Ta) within the range 34-46 degrees C. 2. All embryos survived the 5 hr exposure to Ta of 42 degrees C. The lethal Ta ranged from 44 degrees C (EP eggs) to 46 degrees C (young embryos). 3. The temperature coefficient (Q10) of fH became smaller than 2 in EP eggs as Ta was decreased or increased from 38 degrees C, implying an incipient homeothermic response of fH.