Incubation of cultured bovine vascular smooth muscle cells (VSMC) with forskolin increased cAMP as measured by an increase in cAMP-dependent protein kinase (PKA) activation (PKA ratio). Forskolin also produced a concentration- and time-dependent increase in activity (3-5-fold within 15 min) of a PDE4 (cAMP-specific cyclic nucleotide phosphodiesterase). The increase in PDE4 activity was not affected by cycloheximide and thus not likely due to increased synthesis of the enzyme. Activation, which was preserved during partial purification of the enzyme by chromatography on Sephacryl S-200 and MonoQ, was most likely due to a covalent modification. Incubation of cell homogenates with the catalytic subunit of PKA (PKA(c)) induced a approximately 5-fold activation of PDE4 with a time course similar to that in intact cells after forskolin addition. The forskolin-mediated activation was reversed during incubation of homogenates at room temperature for two hours. Addition of PKA(c) resulted in rapid reactivation of PDE4. These data are consistent with the hypothesis that rapid, reversible activation of PDE4 in cultured VSMC is mediated by PKA.