Fasting heat production (FHP) of growing animals is indicative of their basal metabolic rate and it is proportional to the metabolic body weight (MBW), calculated as BW raised to a certain exponent. lt can be estimated from the analysis of the decreasing kinetic of total heat production during a rather short period of feed deprivation (about one day), as the horizontal asymptotic value corrected for zero physical activity. Specifie exponents should be used to calculate MBW in growing animals over the growing period; a compilation of our data suggest 0.60 in pigs, 0.70 in broilers, 0.75 in turkeys and 0.85 in calves. Therefore, they may differ from the classical 0.75 exponent more adapted to adults. From measurements conducted at different feeding levels, it appears that FHP varies by 0.13, 0.14 and 0.22 kJ per kJ variation in metabolisable energy intake prior to the fasting period, in turkeys, pigs and caIves, respectively. The size of the visceral organs and its evolution during growth would explain the difference between species and the effect offeeding level on basal metabolic rate. Within species, differences in FHP between breeds can be attributed to differences in visceral and protein mass, whereas differences between sexes were only significant when animals approach sexual maturity. Finally, high ambient temperature is associated with decreased FHP that can be mainly explained by the anorexie effect of heat stress. To conclude, variations in FHP arc indicative of variations in maintenance energy requiremcnts that should be taken into account in nutritional recommendations.