1. Readings were made on the rates of oxygen consumption and on the activities of the succinoxidase system of eggs of the mealworm for each day of embryonic development at 30°C.
2. The rate of oxygen consumption, expressed as microliters/50 eggs/hour, was low (4.89) in newly laid eggs. It rose to 7.41 during the next 24 hours, remained at this level for the next 2 days, and then increased during the remainder of the embryonic period reaching a high value of 14.79 at the time of hatching.
3. The activity of cytochrome oxidase in eggs from newly emerged beetles, expressed as Δ log [Cy Fe++]/minute, remained at a value of 0.042 during the first half of the embryonic period, increasing to 0.233 during the latter half of this period.
4. The activity of succinic dehydrogenase showed the same series of changes except at much lower values. Expressed as Δ log [Cy Fe+++]/minute, they ranged from 0.010 in the newly laid egg to 0.034 at the end of the embryonic period.
5. The activity of cytochrome oxidase of the egg was found to decrease with parental age. Eggs from newly emerged beetles had activity values considerably higher than those of beetles 6 or 8 weeks after emergence. However, no comparable changes were noted in the activity of succinic dehydrogenase or in the rate of oxygen consumption. These observations suggest that cytochrome oxidase is not a rate-limiting enzyme in the respiratory metabolism of the mealworm egg.