Abstract Ribonucleotide reductase activity has been measured during the course of one cell-division cycle in a synchronous culture of Saccharomyces cerevisiae. Enzyme activity was distinctly periodic, rising from an initial low level to a maximum early in the S phase, then declining at the end of this phase. The increase in ribonucleotide reductase activity began just prior to the onset of DNA synthesis and coincided with the appearance of buds on the yeast cells. Cycloheximide added at various times during the cell cycle blocked the increase (but not the decline) of ribonucleotide reductase activity; it also inhibited DNA synthesis and bud development. 5-Fluorouracil inhibited DNA synthesis and prevented bud maturation (but not initiation or growth); however, it stimulated an increase in ribonucleotide reductase activity for prolonged periods. Uracil reversed the inhibitory effects of fluorouracil, but not its enhancement of ribonucleotide reductase activity.