With Northern blot analysis, we demonstrate that phorbol 12-myristate 13-acetate (PMA) stimulated P-33 messenger RNA (mRNA)-accumulation in rat hepatoma cells in a time- and concentration-dependent manner similar to insulin and plant lectins. No effect of PMA on P-33 mRNA half-life was detected when mRNA synthesis was inhibited with either actinomycin D or 5.6-dichloro-l-β-D-ribofuranosyl benzimidazole. The effects of insulin and PMA were additive at submaximal concentrations and no additivity was observed under these conditions at maximal concentrations. Thus PMA has a marked insulin-like effect on the accumulation of P-33 mRNA in rat hepatoma cells. © 1987 by The Endocrine Society.