The role of myometrial β-adrenoreceptors (characterized by specific binding of (-)-[3H]-dihydroalprenolol) in the control of uterine relaxation and cAMP production was investigated by determining the concentrations of isoproterenol which produced half the maximal relaxation and half the maximal cAMP production, respectively. Under optimal but different conditions for each, half-maximal concentrations for both agonist-binding to β-adrenoreceptors and cAMP production by intact muscle strips were 20 times greater than half-maximal relaxation. Coupling between theβ -adrenoreceptor and adenylate cyclase was characterized in cell-free preparations where, as previously reported (Krall and Korenman, 1979), enzyme stimulation by the agonist had an absolute guanylyl nucleotide requirement. There was good agreement between the isoproterenol requirements for half-maximal hormone stimulation of adenylate cyclase and half-maximal (3-adreno-receptor binding of the agonist in the presence of guanylyl nucleotide. Thus, in myometrium,β -adrenoreceptors were directly coupled to cAMP production and tightly coupled to relaxation through a highly efficient mechanism. As a consequence, occupancy of less than 10% of the receptors and a commensurately small elevation in cAMP levels could account for one-half of the maximal relaxing effects of isoproterenol.