Dark oxygen uptake was measured manometrically for cells of green high-temperature alga, Chlorella 7-11-05, separated from nonsynchronized populations by centrifugation into fractions of predominantly small or large cells. In the presence of exogenous glucose, respiration activity of the smaller (younger) cell fraction was invariably higher than that of the larger (older) cell fraction. In the absence of exogenous substrate, the difference in respiration rates in two fractions of cells was inconsistent from one experiment to another both in size and in sign. The dependence of dark respiration on the amount of available substrate makes the endogenous respiration rate unsuitable as an indicator of the inherent capacity of respiratory mechanisms. In observations on synchronized heterotrophically grown cells, the glucose respiration rate expressed per dry weight of cells gradually declined over the developmental period irrespective of the adequate exogenous supply of glucose or illumination by weak light. Observations on synchronized heterotrophically grown Chlorella cells thus corroborated studies of glucose respiration in cells separated into are groups by centrifugation. The decline in metabolic activity in the course of cell development previously established for growth and photosynthesis extends to include respiration activity. Disagreements among several investigators in regard to the course of respiration during cell development are probably due to the effects of accessory factors such as strong light during the preceding growth period or the scarcity of respiratory substrate during respiration measurements which affect and distort changes in the inherent capacity of metabolic mechanisms in the course of cell development.