Summary: BHK cells were synchronized by excess thymidine treatment which rested in approximately 90% synchrony. The activity of ornithine decarboxylase (ODC), the rate‐limiting enzyme in polyamine biosynthesis, elevated in early S phase, decreased in G2 + M and G1 phase and then increased during late G1∼ second round of early S phase. The concentration of cyclic adenosine‐3′‐5’monophosphate (cAMP) gradually decreased during SG2 + M phase and then increased during late G1 second round of early S phase. preceding that of ODC activity. The data Suggest that ODC activity might be regulated by cellular cAMP level.