Elevation of intracellular cyclic AMP (cAMP) levels by incubation of intact LRM55 astroglial cells with 0.1 mM forskolin or 0.1 μM isoproterenol (IPR) caused a rapid increase in soluble cAMP phospho-diesterase (PDE) activity. Activation did not require de novo protein synthesis and reached a maximum of ⩾100% increase over basal PDE activity after 15 min of treatment. The increase in activity was recovered in a single peak (peak 3) following DEAE chromatography; the other two peaks separated by this procedure showed no change. Peak 3 had all the characteristics of PDE IV: it was sensitive to rolipram, was insensitive to CI-930 and cyclic GMP (cGMP), had a high affinity for cAMP ( K m ∼- 4 μM), and had a very low affinity for cGMP ( K m > 100 μM). Forskolin treatment resulted in an increase of the V max of peak 3 without affecting its K m . In vitro treatment of peak 3 with the catalytic subunit of protein kinase A increased activity, whereas treatment with alkaline phosphatase decreased activity. The rapid activation of this specific PDE in response to forskolin and IPR represents a novel regulation of PDE IV by a mechanism that seems to involve its phosphorylation by a cAMP-dependent protein kinase.