Cyclins are the essential regulatory subunits of cyclin-dependent protein kinases. They accumulate and disappear periodically at specific phases of the cell cycle. Here we investigated whether variations in cyclin mRNA levels in exponentially growing cells can be attributed to changes in mRNA stability. Mouse EL4 lymphoma cells and 3T3 fibroblasts were synchronized by elutriation or cell sorting. Steady-state levels and degradation of cyclin mRNAs and some other cell cycle related mRNAs were measured at early G1, late G1, S and G2/M phases. In both cell lines mRNAs of cyclins C, D1 and D3 remained unchanged throughout the cell cycle. In contrast, cyclin A2 and B1 mRNAs accumulated 3.1- and 5.7-fold between early G1 and G2/M phase, whereas cyclin E1 mRNA decreased 1.7-fold. Mouse cyclin A2 and B1 genes, by alternative polyadenylation, gave rise to more than one transcript. In both cases, the longer transcripts were the minor species but accumulated more strongly in G2/M phase. All mRNAs were rather stable with half-lives of 1.5-2 h for cyclin E1 mRNA and 3-4 h for the others. Changes in mRNA stability accounted for the accumulation in G2/M phase of the short cyclin A2 and B1 mRNAs, but contributed only partially to changes in levels of the other mRNAs.