The catalytic and guanine nucleotide regulatory (G/F) units of solubilized bovine brain adenylate cyclase were separated by gel filtration as described by Strittmatter, S., and Neer, E. J. ((1980) Proc. Natl. Acad. Sci. U. S. A. 77, 6344-6348). The isolated catalytic unit is activated 4 +/- 1-fold (n = 11) by pure bovine brain calmodulin and is stabilized by calmodulin against thermal inactivation. The separated G/F unit can be freed of endogenous calmodulin by gel filtration in buffer containing 1 mM EDTA and no divalent cations. The calmodulin-free G/F unit still activates the catalytic unit. Re-addition of calmodulin does not affect the rate or extent of activation of the G/F unit by guanosine 5'-(beta, gamma-imino)triphosphate. The activation by calmodulin and the G/F unit together is additive, not synergistic. These studies show that calmodulin interacts with the adenylate cyclase catalytic unit but does not seem to affect the function of the G/F unit.