[Formula: see text] was measured by an open-flow methodology at embryonic ages E3, E7, E11, E15, E19 and at E20 at the internal (IP) and external pipping (EP) phases, at the normal incubation temperature (Ta=38°C), in hypothermia (Ta=30°C) and in hyperthermia (Ta=41 and 44°C).  In the cold, Q10 averaged ~2 at all ages, except in IP and EP when lower values (~1.5) indicated some degree of thermogenesis.  In hyperthermia (38-44°C) Q10 was between 1 and 1.4.  Hyperthermia had no significant effects on [Formula: see text] whether the results combined all ages or considered individual age groups, except in IP (in which [Formula: see text] increased 8% with 44°C) and EP embryos (+13%).  After opening the air cell, which exposed the embryo to a higher O2 pressure, hyperthermic [Formula: see text] was significantly higher than in normothermia in E19 (+13%), IP (+22%) and EP embryos (+22%).  We conclude that in chicken embryos throughout most of incubation neither heat nor oxygen availability limits the normal (normoxic-normothermic) values of [Formula: see text] . 
THERM