Human heart failure is associated with a diminished contractile response to 0-adrenergic agonists. We hypothesized that alterations in the activity of a guanine nucleotide-binding regulatory protein (G protein) might be partially responsible for this abnormality. We therefore measured the activity of G proteins in failing human myocardium utilizing bacterial toxin-catalyzed ADP ribosylation. The activity of a 40,000-mol wt pertussis toxin substrate (aG40) was increased by 36% in failing human hearts when compared with nonfailing controls. In contrast, there was no change in the level of the stimulatory regulatory subunit (G.). The increased activity in aG40 was associated with a 30% decrease in basal as well as 5'-guanylyl imidodiphosphate-stimulated adenylate cyclase activity. These data suggest that increased aG40 activity is a new marker for failing myocardium and may account at least in part for the diminished responsiveness to 01-adrenergic agonists in the failing human heart.