During a large portion of embryonic development oxygen consumption (V(O2)) is mainly needed for maintenance (including organ function) and for growth. In an attempt to evaluate their relative contribution, we followed the assumption that the former is proportional to embryo's weight (W) and the latter to growth rate (GR), as V(O2)=aW+bGR, a and b being the respective proportionality coefficients. Fertile chicken eggs were incubated at the normal temperature (38 degrees C) and at various low-temperature conditions, which invariably decreased GR. At a few days intervals over the embryonic ages E9-E18, the W and V(O2) of the embryos were measured and GR computed from the age-W relationship. These simultaneous sets of W (g), GR (g/day) and V(O2) (ml/day) were used to solve the equation; a averaged 14.7 ml O(2) g(-1) day(-1) and b averaged 41.1 ml O(2) g(-1). Hence, over the period of incubation studied, the cost of growing 1g of tissue averaged about 3 times the cost of maintaining it. These coefficients were not significantly different whether computed during normal growth at 38 degrees C or during incubation at low temperatures, indicating that cooling only decreased GR and the cost required by it. Embryos incubated in hypoxia (15% O(2)) had low V(O2) originated not only by the decreased GR but also by a drop in the cost of maintenance, especially in the youngest embryos.