Hydroxyurea treatment of onion roots induced mitotic block which was released by transfer of bulbs to water, and also to some extent by addition of cold or 3H-thymidine to hydroxyurea solutions. In presence of hydroxyurea there was noted very intense incorporation of 3H-thymidine into cell nuclei, giving labelling index of 40-70%. However, all the mitotic figures appearing in presence of hydroxyurea and 3H-thymidine were unlabelled. On the other hand, labelled mitotic figures were obtained when roots incubated with 3H-thymidine in presence of hydroxyurea had been transferred to water. Incorporation of 3H-uridine was unaffected by hydroxyurea. The results show that hydroxyurea arrests onion root meristematic cells, either in the S phase and the G2 phase. Enhanced incorporation of 3H-thymidine in the presence of hydroxyurea, and release by added thymidine of the mitotic block indicate that hydroxyurea induces in onion root meristematic cells a particular shortage of thymidylate.