Abstract: Findings of increased Gsα levels and forskolin-stimulated adenylyl cyclase activity in selective cerebral cortical postmortem brain regions in bipolar affective disorder (BD) implicate increased cyclic AMP (cAMP)-mediated signaling in this illness. Accumulating evidence suggests that intracellular levels of cAMP modulate the abundance and disposition of the regulatory subunits of cAMP-dependent protein kinase (cAMP-dPK). Thus, in the present study, we tested further whether hyperfunctional Gsα-linked cAMP signaling occurs in BD by determining [3H]cAMP binding, a measure of the levels of regulatory subunits of cAMP-dPK, in cytosolic and membrane fractions from discrete brain regions of postmortem BD brain. Specific [3H]cAMP (5 nM) binding was determined in autopsied brain obtained from 10 patients with DSM-III-R diagnoses of BD compared with age- and postmortem delay-matched controls. [3H]cAMP binding was significantly reduced across all brain regions in cytosolic fractions of BD frontal (−22%), temporal (−23%), occipital (−22%) and parietal (−15%) cortex, cerebellum (−36%), and thalamus (−13%) compared with controls, but there were no differences in [3H]cAMP binding in the membrane fractions from these same regions. These results suggest that changes occur in the cAMP-dPK regulatory subunits in BD brain, possibly resulting from increased cAMP signaling. The possibility that antemortem lithium and/or other mood stabilizer treatment may contribute to the above changes, however, cannot be ruled out.