Partial submergence or treatment with either ethylene or gibberellin (GA) promotes rapid internodal growth in deepwater rice (Oryza sativa L.). Earlier work has shown that GA is the immediate hormonal signal for this growth response, which involves induction of the cell cycle at the G2/M phase transition and subsequent enhancement in the rate of DNA synthesis. In all eukaryotes, onset of mitosis is regulated by the p34cdc2/CDC28 protein kinase, whose activity is assayed by in vitro phosphorylation of histone H1. It was found that GA enhanced the activity of p34cdc2/CDC28-like histone H1 kinase in the intercalary meristem of rice internodes. The enzyme activity showed a sharp peak that correlated with a decrease in the population of cells in the G2 phase during the first 4 h of GA treatment but not with changes in DNA synthesis. The level of histone H1 kinase activity increased again when cell division activity in the intercalary meristem is known to be high. The expression of two cdc2 homologs was examined. The mRNA level of one of these, cdc2Os-2, was increased after 1 h of GA treatment, whereas the mRNA level of the other, cdc2Os-1, was not affected. Two cDNAs, cycOs1 and cycOs2, which show high homology to cyclin cDNAs, were cloned from rice. They share 75.1% sequence identity at the amino acid level, and both of them are encoded by mRNAs of 1.6 kb. Expression of the two corresponding cyclin genes was enhanced by GA, and the time course of the induction was compatible with a role for both cyclins in regulating the G2/M phase transition. The cyclins were expressed in the intercalary meristem and the elongation zone of the internode, but the GA-induced increase in transcript levels was restricted to the meristem only. The results support the hypothesis that induction of mitosis by GA is brought about by increased p34cdc2/CDC28 protein kinase activity, which may be the result of transcriptional activation of the cdc2Os-2, cycOs1 and cycOs2 genes.