Excised pea-root tips were incubated for 4 h in gas mixtures containing 0$\cdot $00001 to 100% oxygen, in order to determine the effect upon mitosis. Below 0$\cdot $0005% oxygen, mitosis was completely arrested. Between 0$\cdot $001 and 0$\cdot $2% oxygen, cells initially in mitosis completed division, but no more cells started dividing. Between 0$\cdot $05 and 0$\cdot $2% oxygen, cells initially in interphase entered division, but did not finish. Above 0$\cdot $5% oxygen, all cells not prevented from dividing by excision finished division within 4 h. After exposure to 0$\cdot $05% oxygen for 4 h, an excessive proportion of cells was found in prophase; in 0$\cdot $1% oxygen an excess of metaphases, and in 0$\cdot $2% oxygen an excess of telophases resulted. The oxygen uptake and carbon dioxide output of root tips were measured in a range of oxygen tensions and in anaerobic conditions. The relationship between oxygen uptake and oxygen tension was hyperbolic; a half maximum rate of oxygen uptake was obtained at about 10% oxygen. It was concluded that the respiration of root tips was limited by slow diffusion of oxygen through the tissue. From the carbon dioxide output it was estimated that the amount of energy available to isolated root tips under anaerobic conditions was about 1% of that available under aerobic conditions. Possible mechanisms whereby extreme oxygen-lack could arrest mitosis were considered. It was shown that the arrest was not due to abolition of a gross supply of energy. No evidence was obtained as to what other mechanism might be operative. An hypothesis was formulated in an attempt to explain the complicated relationship between mitosis and oxygen tension. It was assumed that the visible phases of mitosis are immediately preceded by a phase with a higher requirement for oxygen than mitosis, and that preceding this is an earlier phase with a lower oxygen requirement than mitosis.