The beta-adrenoceptor-adenylyl cyclase system was studied in heart ventricles from Wistar rats, cardiomyopathic (BIO 8262) and nonfailing control hamsters (CLAC) using the beta 1-adrenoceptor antagonist CGP 20712A. In radioligand binding studies, the majority of beta-adrenoceptors in ventricles from rats as well as from CLAC hamsters was of the beta 1-subtype (72.2% and 76.6%, respectively). In BIO ventricles a significant (CLAC vs. BIO, P < 0.05) reduction in the beta 1-subtype (62.9%) was found. In Wistar rats the subtype-mediated stimulation of adenylyl cyclase reflected the beta 1:beta 2 ratio as determined by binding studies. In hamster ventricles the effect of isoprenaline was mediated predominantly (CLAC) or exclusively (BIO) via the beta 2-subtype, indicating that cardiac beta 1-adrenoceptors were partly (CLAC) or completely (BIO) uncoupled from the adenylyl cyclase.