In end-stage heart failure, cardiac beta-adrenoceptors are decreased and cardiac Gi protein is increased. We assessed beta-adrenoceptors, G proteins, and effects of several beta-adrenoceptor agonists, histamine, and 5-HT on adenylyl cyclase activity in right and left atria and left ventricles and on left ventricular contractility in six potential heart transplant donors (nonfailing hearts; NFHs) and in nine patients with end-stage dilated cardiomyopathy (DCM) and 11 patients with end-stage ischemic cardiomyopathy (ICM) to establish whether the functional responsiveness of all cardiac Gs-coupled receptors is reduced. Beta-adrenoceptors were reduced in all three tissues; in DCM, beta1-adrenoceptors were more markedly downregulated; in ICM, both beta1- and beta2-adrenoceptors were diminished. In all three tissues, isoprenaline-, terbutaline-, histamine- and 5-HT-induced adenylyl cyclase activation was reduced similarly in DCM and ICM. Moreover, in DCM and ICM, guanosine triphosphate (GTP)- (involving Gs and Gi) activated adenylyl cyclase was significantly diminished, whereas NaF-activated (involving only Gs) and Mn2+-activated (acting at the catalytic unit of the enzyme) adenylyl cyclase was unaltered. Left ventricular positive inotropic responses to beta1- (noradrenaline, dopamine, and dobutamine), beta2- (terbutaline), and beta1- and beta2-adrenoceptors (isoprenaline, adrenaline, and epinine), as well as H2-receptor (histamine) stimulation were significantly reduced. The extent of reduction was not different for each agonist in ICM and DCM. We conclude that in DCM and ICM, functional responsiveness of all cardiac Gs-coupled receptors is similarly reduced.