Due to its small body size, which in the case of prematurity is associated with increased skin permeability and incomplete brown fat development, the human neonate is highly prone to heat loss. Hence, thermal protection is mandatory to prevent neonatal hypothermia with its adverse metabolic and hemodynamic effects. This is usually attempted by the selection of "thermoneutral" ambient temperatures to maintain core temperature constant without regulatory metabolic increase. Since, however, core temperature decreases only after metabolic increase has failed to counteract heat loss, and metabolic increase in its turn is preceded by peripheral vasoconstriction, recording of thermal gradients between core and peripheral temperatures has now proven superior in early detection of thermal stress and maintenance of thermal comfort in the neonate. The longlasting thermal lability of preterm babies partly results from the fact that an elevated basal metabolic rate, which in term neonates compensates for the small body size, is only achieved with delay. As this is correlated to the growth retardation typical of prematurity, the postnatal metabolic increase is usually considered to be a precondition for growth. However, a comparative calorimetric investigation has revealed thar in a marsupial species normally born in a very immature state, a rapid weight increase occurs at a low metabolic rate. Obviously, these animals retain a growth efficiency which in humans is confined to intrauterine life and interrupted by preterm birth. Moreover, their low O-2 consumption rate is adaptive to restricted respiratory surface area and incomplete tissue vascularization and contributes to hypoxia tolerance. Therefore, although postnatal metabolic increase promotes thermal stability and weight increase in term and slightly preterm human neonates, metabolic reduction may be the more appropriate strategy in cases where O-2 and substrate supply are limited by extreme immaturity. As long as the factors mediating natural metabolic suppression are unknown, careful thermal protection seems to be one of the most promising ways to prevent uneconomic metabolic activation in highly preterm human neonates.