We have recently demonstrated that the beta subunit of the heterotrimeric G-proteins is endogenously mono-ADP-ribosylated in intact cells. The modified betagamma heterodimer loses its ability to inhibit calmodulin-stimulated type 1 adenylate cyclase and, remarkably, is de-ADP-ribosylated by a cytosolic hydrolase that completes an ADP-/de-ADP-ribosylation cycle of potential physiological relevance. In the present study, we show that this ADP-ribosylation might indeed be a general mechanism for termination of betagamma signalling, since the ADP-ribosylated betagamma subunit is also unable to activate both phosphoinositide 3-kinase-gamma and phospholipase C-beta2. Moreover, we show that beta subunit ADP-ribosylation is induced by G-protein-coupled receptor activation, since hormone stimulation of Chinese-hamster ovary plasma membranes leads to increases in beta subunit labelling. This occurs when betagamma is in its active heterodimeric conformation, since full inhibition of this modification can be achieved by binding of GDP-alphai3 to the betagamma heterodimer. Taken together, these findings delineate a pathway that arises from the activation of a G-protein-coupled receptor and leads to the inhibition of betagamma activity through its reversible mono-ADP-ribosylation.