Catecholamine stimulation of adenyl cyclase in plasma membranes from rat myometrium was typical of beta-adrenoceptor stimulation. Thus, isoproterenol was the most potent followed by epinephrine and norepinephrine; phenylephrine, a pure alpha-agonist, did not activate the enzyme. The catecholamine-induced activation was potently inhibited by propranolol, whereas phenozybenzamine was without effect. The ED50 for 1-isoproterenol activation was 10- minus 7 M. Binding of [3H]-d-isoproterenol to the same preparations bore little resemblance to the pattern of 1-isoproterenol activation of adenyl cyclase. Propranolol, even at high concentrations, was without significant effect. The degree of binding was inversely rflated to the osmolarity suggesting that diffusion may have contributed to the binding. This contention was further supported by the time course of binding which was biphasic- the first component (smaller than 4 min) being less affected by osmolarity than the second (greater than 4 min). The optimum pH for binding was 7.4.