Diethyldithiocarbamate (DDTC) injected i.p. inhibits remarkably and in a dose-dependent manner 12-O-tetradecanoylphorbol-13-acetate (TPA)-decreased glutathione (GSH) peroxidase and TPA-induced ornithine decarboxylase (ODC) activities in mouse epidermis in vivo. DDTC is more potent in inhibiting these effects of TPA than 16 other antioxidants, free radical scavengers, thiol-containing compounds, and reduced glutathione (GSH) level-raising agents, even though some of these treatments are applied directly to the TPA-treated skin. DDTC also inhibits the effects of several structurally different tumor promoters and the greater GSH peroxidase and ODC responses produced by repeated TPA treatments. The inhibitory effects of DDTC on TPA-decreased GSH peroxidase and TPA-induced ODC activities are additive with those of Na2SeO3 and D-alpha-tocopherol (vitamin E). Interestingly, DDTC is a more effective inhibitor when it is administered after TPA, suggesting that DDTC may supplement, facilitate, and/or enhance the activity of the natural GSH-dependent detoxifying system protecting the epidermis against the oxidative challenge presumably linked to the tumor-promoting activity of TPA. When tested in the initiation-promotion protocols, DDTC inhibits to the same degree complete tumor promotion by TPA and stage 2 tumor promotion by mezerein, in relation with its identical inhibition of the GSH peroxidase and ODC responses to both TPA and mezerein. Moreover, the inhibition of the first stage tumor-promoting activity of TPA by DDTC may be attributed to its ability to inhibit TPA-induced DNA synthesis, a postulated component of the conversion phase of skin carcinogenesis when TPA is used as a stage 1 tumor promoter.