Troglitazone is one of the thiazolidinedione (TZD) class of anti-diabetic drugs and a ligand for peroxisome proliferator-activated receptor gamma (PPARgamma). Troglitazone and other PPARgamma ligands have been shown to inhibit cell proliferation and induce cell cycle arrest in a variety of cancer cells, and have been considered as potential tumor preventive and tumor therapeutic agents. Little is known, however, about how normal or initiated cells respond to these agents during mouse skin carcinogenesis. We report here that troglitazone and another TZD, ciglitazone, dramatically inhibited mitogen-induced cellular proliferation in normal mouse skin primary keratinocytes and in the C50 keratinocyte cell line. This was accompanied by induction of cell cycle G1 phase arrest and suppression of cyclin D1, cdk4, and cdk2 expression. Troglitazone suppressed cyclin D1 expression at multiple levels. In addition, we demonstrated that PPARgamma was not expressed at functional levels in cultured mouse skin keratinocytes, and that the inhibitory effects of troglitazone on cellular proliferation and cyclin D1 expression in these cells were PPARgamma-independent. Given the important role of keratinocyte proliferation in skin carcinogenesis, our data suggest that TZD may be useful tumor preventive agents in skin.