The production of heat (or thermogenesis) and its response to cold improve very quickly around birth in both mammals and birds. The mechanisms for such rapid perinatal development are not fully understood. Previous experiments with hyperoxia suggested that, during the last phases of incubation, eggshell and membranes might pose a limit to oxygen availability. Hence, it was hypothesized that an improvement in oxygenation by opening the eggshell may contribute to the establishment of thermogenesis. Thermogenesis and its response to cold were measured by indirect calorimetry, in warm (38 degrees C) conditions and during 1-h exposure to 30 degrees C. Both improved throughout the various phases of the hatching process. During the latest incubation phases (internal pipping, IP, and star fracture of external pipping, EP), the removal of the eggshell in the region above the air cell raised metabolic rate both in warm and cold conditions (in IP) or the thermogenic response to cold (in EP). Adding hyperoxia after opening the eggshell caused no further increase in the thermogenic response. In cold-incubated embryos thermogenesis during the EP phase was much less than normal; in these embryos, increasing the oxygen availability did not improve thermogenesis. We conclude that oxygenation contributes to the maturation of the thermogenic mechanisms in the perinatal period as long as these mechanisms have initiated their normal developmental process.