In this study we report a new mechanism whereby cyclic AMP (cAMP) regulates the cell-cycle machinery. We demonstrate that elevation of intracellular levels of cAMP promotes degradation of cyclin D3 in proteasomes, and that this occurs via glycogen synthase kinase-3β (GSK-3β)-mediated phosphorylation of cyclin D3 at Thr-283. Elevation of cAMP did not change the subcellular distribution of either cyclin D3 or GSK-3β. However, cAMP promoted the interaction between cyclin D3 and GSK-3β both in vitro and in vivo, indicating that GSK-3β-mediated phosphorylation of cyclin D3 might require the association between the two proteins. These results demonstrate how cAMP enhances degradation of cyclin D3. Furthermore, we provide evidence for a novel mechanism by which GSK-3β might phosphorylate unprimed substrates in vivo.