all-trans retinoic acid (atRA), the oxidative metabolite of vitamin A, is essential for normal embryonic development. Also, high levels of atRA are teratogenic in many species and can effectively induce cleft palate in the mouse. Most cleft palate resulted from the failed fusion of secondary palate shelves, and maintenance of the normal cell proliferation is important in this process of shelf growth. Toclarify the mechanism by which atRA causes cleft palate, we investigated the effect of atRA on proliferation activity and cell cycle distribution in mouse embryonic palatal mesenchymal (MEPM) cells. atRAinhibited the growth of MEPM cells by inducing apoptosis in a dose-dependent manner. atRA also caused aG1 block in the cell cycle with an increase in the proportion of cells in G0/G1 and a decrease in the proportion of cells in S phase, as determined by flow cytometry. We next investigated the effects of atRA on molecules that regulate the G1 to S phase transition. These studies demonstrated that atRA inhibited expression of cyclins D and Eat the protein level. Furthermore, atRA treatment reduced phosphorylated Rb and decreased cdk2 and cdk4 kinase activity. These data suggest that atRA had antiproliferative activity by modulating G1/S cell cycle regulators and by inhibition of Rbphosphorylation in MEPM cells, which might account for the pathogenesis of cleft palate induced by retinoic acid.