Cell cycle progression is tightly regulated by cyclins, cyclin-dependent kinases (cdks) and related inhibitory phophatases. Here, we employed mitotic selection to synchronize the C6 glioma cell cycle at the start of the G1 phase and mapped the temporal regulation of selected cyclins, cdks and inhibitory proteins throughout the 12 h of G1 by immunoblot analysis. The D-type cyclins, D3 and D1, were differentially expressed during the C6 glioma G1 phase. Cyclin D1 was up-regulated in the mid-G1 phase (4-6 h) while cyclin D3 expression emerged only in late G1 (9-12 h). The influence of the anticonvulsant agent valproic acid (VPA) on expression of cyclins and related proteins was determined, since its teratogenic potency has been linked to cell cycle arrest in the mid-G1 phase. Exposure of C6 glioma to VPA induced a marked up-regulation of cyclin D3 and decreased expression of the proliferating cell nuclear antigen. In synchronized cell populations, increased expression of cyclin D3 by VPA was detected in the mid-G1 phase (3-5 h). Immunocytochemical localization demonstrated rapid intracellular translocation of cyclin D3 to the nucleus following VPA exposure, suggesting that VPA-induced cell cycle arrest may be mediated by precocious activation of cyclin D3 in the G1 phase.