Abstract-The teratogenic mechanism(s) of valproate (VPA) have been suggested to anse through inhibition of proliferation coupled with differentiation at a mid-G I phase restriction point in the cell cycle. As protein kinase C (PKC) plays a pivotal role in cell proliferation and differentiation, the effect of inhibitors specific for the catalytic and regulatory domains on transit through the G I phase of the cell cycle was determined. Calphostin C and bisindolylmaleimide GF 109203X produced a dose-dependent decrease in proliferation of C6 glioma with approximate 50% inhibitory concentration values of 10 nM and 111M, respectively. Flow cytometric analysis indicated proliferative arrest to be in the G I phase with the expected concomitant decrease of cells in the G2/M and S phases. Following release from drug-induced proliferative arrest, cells exhibited a synchronous entry into S phase as evidenced by an increase in [3H)thymidine incorporation after approximately 6-8 hr, indicating the restriction point to be in the mid-GI phase. Using mitotically synchronized cells continuously exposed to valproate (2 mM), PKC activity was found to be significantly reduced in the mid-G I phase (5.5 hr) but not at an earlier (2.5 hr) time point, implying VPA to exert its effect at a site upstream to the point of proliferative arrest at 5-6 hr into the G I phase which as yet, remains to be defined. er) 1998 Elsevier Science Ltd Abbreviations: ANOVA = analysis of variance; DMEM = Dulbecco's modified Eagle's medium; DMSO = dimethyl sulfoxide; IC so = concentration at which 50% inhibition is achieved; PBS = phosphate buffered saline; PKC = protein kinase C; PLC = inositol phospholipid-specific phospholipase C; VPA = valproic acid.