The insulin-like effects of vanadiumin vivo are likely to be achieved at micromolar concentrations. Demonstrated effects of vanadium on adipose tissue of streptozotocin-diabetic rats include inhibition of basal and stimulated rates of lipolysis and effects on fat cell protein phosphorylation. The studies described below examined the effects of vanadium (to a maximum concentration of 0.5 mM) on adipose cells or tissuein vitro. Vanadium, added as a vanadyl-albumin complex or as sodium orthovanadate, produced a marked (greater than 50%) inhibition of isoproterenol-stimulated lipolysis. Inhibition of lipolysis equivalent to that seen with insulin, was achieved with ∼100 μM vanadium. In contrast, no insulin-like stimulation ofde novo fatty acid biosynthesis was observed with vanadium below 0.5 mM. Surprisingly, the antilipolytic effects of vanadium persisted in the presence of cilostamide, an inhibitor of the insulin-sensitive isoform of cyclic nucleotide phosphodiesterase. Studies with purified preparations of the catalytic subunit of cyclic AMP-dependent protein kinase revealed dose-dependent inhibition with vanadyl-glutathione (to a maximum of ∼40% inhibition). Equivalent inhibition of cyclic AMP-dependent phosphorylation of Kemptide (∼50%) was observed upon incubation of freshly-prepared fat-pad supernatant fractions with vanadyl-glutathione. These results suggest that effects of low concentrations of vanadium may be mediated, at least in part, by actions on the catalytic subunit of cyclic AMP-dependent protein kinase.