The tumor promters, 12-O-tetradecanoylphorbol-13-acetate (TPA), phorbol-12,13-didecanoate, teleocidin, and dihydroteleocidin, at nM levels, but not the non-tumor-promoting 4.alpha.-phorbol-12,13-didecanoate even at micromolar concentrations, stimulated arachidonic acid metabolism in cultured rat liver cells. These liver cells synthesize primarily prostaglandin(PG)I2 (measured as its nonenzymatic hydrolytic product, 6-keto-PGF1.alpha.). The production of 6-keto-PGF1.alpha. increased with time of incubation with TPA and was essentially complete in 4 h. Cycloheximide, at nanomolar levels, blocked the TPA-stimulated 6-keto-PGF1.alpha. production in a dose-dependent manner; this inhibition was related to inhibition of protein synthesis. Chelation of Ca2+ by ethyleneglycol-bis(.beta.-aminoethyl ether)-N,N''-tetraacetic acid, treatment of the cells with the Ca2+ channel blocker, nifedipine, or inhibition of intracellular Ca2+ mobilization by 8-diethylamine)octyl-3,4,5-trimethoxybenzoate hybrochloride also inhibited TPA-stimulated 6-keto-PGF1.alpha. production. The steroidal antiinflammatory drug, dexamethasone, a potent in vivo inhibitor of tumor promotion, was an inhibitor of 6-keto-PGF1.alpha. stimulation by TPA.