Progress through the eukaryotic cell cycle is regulated by cyclin-dependent cdc2 protein kinases. In rice (Oryza sativa L), two cdc2 protein kinases, cdc2Os-1 and cdc2Os-2, and two cyclins, cycOs1 and cycOs2, have been described. In this study, we report on the cell-cycle phase-specific expression of these genes. Using partially synchronized suspension cells from rice, we found that cdc2Os-1 was expressed constitutively throughout the cell cycle. The cdc2Os-2 transcript level was elevated in G1 and S phase. The cycOs1 and cycOs2 transcripts increased steadily through G2 and dropped off rapidly in mitosis as is typical for mitotic cyclins. We hypothesize that the cdc2Os-2 gene product acts in G1/S and that the growth-promoting hormone gibberellin (GA) that induces expression of cdc2Os-2, cycOs1 and cycOs2 in the intercalary meristem of deepwater rice internodes accelerates G1/S phase progression through increased expression of cdc2Os-2 and G2/M phase progression through increased expression of the mitotic cyclins cycOs1 and cycOs2. The R2 gene from rice has 55% sequence identity to the cdc2-activating kinase (CAK) family of protein kinases which have been shown to phosphorylate and thereby activate cdc2 protein kinases in animals and yeast. In partially synchronized suspension cells, R2 mRNA levels were elevated in G1 and S phase. In GA-treated rice internodes, R2 transcript levels were elevated in the meristem and part of the elongation zone. These results are consistent with a role for R2 in regulating G1/S phase progression.