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S U M M A R YLarge-scale synchronous cultures of the fission yeast Schizosaccharot?iyces/ionzbe 972h-were prepared by a sedimentation-velocity selection method in a zonal rotor.Oxygen uptake was measured polarographically on samples withdrawn at frequent intervals from the culture vessel. Rates of oxygen uptake, expressed per ml culture, increased exponentially, doubling over each cell cycle, but rose to maxima twice per cycle, once during cell division and once at approximately one-half of the cycle. Oxygen uptake at the maxima was stimulated by carbonyl-cyanide m-chlorophenyl hydrazone; that at the troughs was insensitive to this compound. Oxygen uptake was inhibited at all points in the cycle by antimycin A and cyanide at low concentrations; inhibition by these compounds did not alter the periodicity of the oscillations.Heat evolution increased at a constant rate during two cycles of synchronous growth, but in the presence of carbonyl-cyanide m-chlorophenyl hydrazone, peaks of heat evolution in phase with peaks of oxygen uptake were observed. The discontinuous respiratory activity of mitochondria through the cell cycle is discussed with reference to previous contrasting data, and possible control mechanisms are suggested .