Abstract: This study examined effects of tubulin on the activation of adenylyl cyclase in rat cerebral cortex membranes. Tubulin, prepared from rat brain by polymerization with the hydrolysis‐resistant GTP analogue 5′‐guanylylimidodiphosphate (GppNHp) caused significant activation of the enzyme by ∼156% under conditions in which stimulation rather than inhibition of the enzyme was favored. Tubulin‐GppNHp activated isoproterenol‐sensitive adenylyl cyclase, potentiated forskolin‐stimulated activity of the enzyme, and reduced agonist binding affinity for β‐adrenergic receptors. When tubulin, polymerized with the hydrolysis‐resistant photoaffinity GTP analogue [32P]P3(4‐azidoanilido)‐P1‐5′‐GTP ([32P]AAGTP), was incubated with cerebral cortex membranes, AAGTP was transferred from tubulin to Gsα as well as Giα. These results suggest that, in rat cerebral cortex membranes, the tubulin dimer participates in the stimulatory regulation of adenylyl cyclase by transferring guanine nucleotide to Gsα, as well as affecting the Gi‐mediated inhibitory pathway.