Regulation of the expression of cAMP-dependent protein kinase in cellular aging was studied using the IMR-90 diploid human lung fibroblasts. The level of cAMP-dependent protein kinase present in cell extracts was monitored by 1) photoactivated incorporation of 8-N3-[32P]cAMP into the 47,000- and 54,000-dalton regulatory subunits of the type I and type II cAMP-dependent protein kinases, respectively; 2) cAMP-dependent phosphorylation of histone II AS catalyzed by the catalytic subunit of the kinase; and 3) fractionation and analysis of the type I and type II cAMP-dependent protein kinase by DEAE-Sephacel column chromatography. Our results showed an approximately two- to threefold increase in the level of the type I cAMP-dependent protein kinase and a somewhat smaller increase in the type II kinase in extracts of the "old" IMR-90 cells (population doubling greater than 48) as compared to that of the "young" cells (PDL 22-27). The timing of the increase in cAMP-dependent protein kinase coincided with a significant decrease in the proliferative potential of the cells. This result together with previously demonstrated effects of cAMP in the control of cell growth and differentiation and the increased expression of cAMP-dependent protein kinase during terminal differentiation of the murine preadipocytes (3T3-L1) and myoblast (L-5, L-6, and C2C13) suggests that regulation of the levels of cAMP and cAMP-dependent protein kinase plays a significant role in the control of cell growth and differentiation.