In the fission yeast Schizosaccharomyces pombe the rate of RNA synthesis, as determined by pulse labeling, increases in a step-like manner in synchronous cultures prepared by centrifugal elutriation. Cultures prepared in this way show marked reductions in perturbations which can be caused by many synchrony techniques. Kinetic evidence indicates that alterations in pool metabolism are not responsible for the step pattern. The long period of increase in the rate doubling (relative to cell number increase) indicates that the period of increasing rate may be due to a growth period and not a sudden transition between a slow and a fast rate. An analysis of synchronous cultures of cells of different cell size and synchronous cultures of temperature sensitive mutants blocked in cell cycle progress indicated that neither size control, changes in DNA content nor septation are directly responsible for the steps in RNA synthesis. Instead the time of the rate change is associated with a specific point in the cell cycle, probably an event associated with nuclear division.