Significant increases in rate of mitosis were found to follow treatment of yeast cells with acute doses of either X-rays (500 r) or UV light (200 ergs/mm2). Mean generation time decreased by as much as 14% over 3 generations. The stimulation was detected after the 2nd post-irradiation division. It was concluded that the increase in mitotic activity is an effect of radiation on cells and is not due to changes in the irradiated medium. The stimulus is transmitted to successive cells but is not a mutation. There is a correlation between length of inhibitory period and degree of later stimulation. However, results suggest that the inhibition does not bear a causal relationship to the stimulus; cells are stimulated when inhibitory influences are absent.