The cdc2 gene encodes a 34 kDa protein $(p34^{cdc2})$ which is a protein kinase and plays a regulatory role in the cell cycle. To study the regulation of cdc2 in regenerating rat liver, we have cloned rat cdc2 cDNA from a rat liver cDNA library. The nucleotide sequence of the coding region, and the deduced amino acid sequence for 297 residues, show, respectively, 94.1% and 99.3% homologies to mouse cdc2. Expression of cdc2 during the first cell cycle of rat liver regeneration after partial hepatectomy was investigated. The level of mRNA was low until entry into S phase, increased to a maximum during S and G2 phase, decreased during M phase, and dropped to the background level upon exit from M phase. The pattern of $p34^{cdc2}$ activity level change was similar to the that of mRNA.