A correlation between phosphorylation and stimulation of RNA polymerases I and II by homologous nuclear protein kinase peak I from small dense nuclei of mouse brain was demonstrated. Incubation of RNA polymerase II with nuclear protein kinase peak I lowered the optimum Mg2+ concentration of the polymerase in the same manner as chronic morphine treatment alone did, suggesting that the change in Mg2+ optimum of RNA polymerase II seen during morphine tolerance-dependence may occur through changes in phosphorylation as a result of increased nuclear protein kinase activity. Experiments investigating the involvement of cyclic 3',5'-adenosine monophosphate (cAMP) in morphine tolerance-dependence demonstrated that dibutyryl-cAMP enhanced the degree of morphine tolerance developed, as reported previously [I. K. Ho, H. H. Loh and E. L. Way, J. Pharmac. exp. Ther. 185, 347 (1973); I. K. Ho, H. H. Loh, H. N. Bhargava and E. L. Way, Life Sci. 16, 1895 (1975)], and also enhanced the chronic morphine-induced increase in nuclear protein kinase specific activity. Alterations in the regulation of the nuclear protein kinase activity may bave subsequently affected RNA polymerase activity through phosphorylation. These results suggest that, during morphine tolerance-dependence development, cAMP may be involved in the possible nuclear protein kinase regulation of homologous RNA polymerase.