To determine if cardiac tissue is capable of modulating its response to a stimulating hormone, we studied desensitization to the positive inotropic effect of catecholamines on embryonic chick ventricular tissue using a phase contrast microscope-video motion detector system and correlated the contractility findings with concurrent observations of β-adrenergic receptor properties and aden-ylate cyclase activity. Incubation for 30 minutes with μIM isoproterenol produced a diminution in the subsequent inotropic response to 0.1μIM isoproterenol to 35± 8% (mean± SEM) of the initial response. This desensitization to the positive inotropic effect of isoproterenol was catecholamine-specific and was not accompanied by alteration in the inotropic response to Ca2+. To investigate the mechanism of desensitization, we studied properties of the β-adrenergic receptor in homogenates of chick embryo ventricle using [3H]dihydroalprenolol as a ligand. Thirty minutes of incubation with 1 pM isoproterenol produced no change in β-adrenergic receptor density (92.8 ± 5.1 fmol/mg protein) and only a small change in receptor affinity (KD= 5.2± 0.3 DM VS. 7.0± 0.3 nM;P< 0.01). Receptor affinity for isoproterenol, as judged by [3H]dihydroalprenolol displacement, was not changed significantly. The adenylate cyclase stimulation by isoproterenol in similarly prepared tissue, however, was reduced to 29% of the control value after 30 minutes of exposure to 1 /IM isoproterenol. Adenylate cyclase sensitivity was restored by guanosine 5'-(β,γ-imino)triphosphate. Thus, desensitization of physiological respon-siveness of ventricular tissue to a β-adrenergic agonist was accompanied by little change in β-adrenergic receptor properties but by marked diminution in adenylate cyclase responsiveness. These observations suggest that uncoupling of the β-adrenergic receptor-adenylate cyclase complex may be the mechanism of short-term desensitization of ventricular muscle to the positive inotropic effects of isoproterenol.Circ Res 47: 493-501,1980