Mesaconitine (MA) significantly stimulated the incorporation of 5-[3H]-orotic acid into liver nuclear RNA 16 h after its administration. MA exhibited the strongest activity among the aconitine alkaloids. This stimulatory effect of MA was inhibited by actinomycin D, but the incorporation ratio of 5-[3H]orotic acid in the combined treatment group with MA and actinomycin D was significantly higher than that in the single treatment group with actinomycin D. MA also increased the incorporation of 5-[3H]orotic acid into polysomal RNA in mouse liver. When MA was added to the RNA polymerase (EC 2.7.7.6) preparation obtained from rat liver, the increase of the enzyme activity was weak. While RNA polymerase preparation from the liver of rats, which were previously treated with MA, elicited increased incorporation of [3H]cytidine monophosphate into RNA as compared with that from the livers of normal rats. Accumulated data indicate that aconitine alkaloids, in particular MA, accelerate liver RNA synthesis mainly by the increase of RNA polymerase.