The phenomena of x-irradiation induced delay of the first mitotic divisions after irradiation and the stimulation of later ones as well as the correlation between them was studied in the progeny of isolated cells of a diploid strain of Saccharomyces cerevisiae. The length of the lag and of the mean generation time was not influenced by a fourfold increase in radiation intensity, but it was strongly dependent on radiation dose: No stimulation at 1000 r and almost no delay, increasing stimulation and delay up to 5000 r, remaining constant up to 6000 r. In general, there was a strong correlation between the length of lag and the stimulation of a clone. But even those clones, which were not delayed, had a shorter mean generation time than the controls. This proves that stimulation is not a consequence of prior inhibition. Postirradiation treatments with mitotic inhibitors (8-Hydroxyquinoline and Mercaptoethanole) showed that delay and stimulation are both dependent on the same primary radiation damage, but each in a different way.