Heterotrimeric G proteins function to relay information from cell surface receptors to intracellular effectors. In mammals, G protein (alpha), (beta) and (gamma) polypeptides are encoded by at least 16, 4 and 7 genes, respectively. Most interest in G proteins has been focused on their (alpha) subunits, since these proteins bind and hydrolyze GTP and most obviously regulate the activity of the best studied effectors. Four G(alpha) GTPase-activating proteins (GAPs) have been identified and are designated RGS1 (regulator of G protein signaling), RGS4, RGS10 and GAIP (G(alpha)-interacting protein). Each of these proteins has been shown to deactivate specific G(alpha) isoforms by increasing the rate at which they convert GTP to GDP. RGS1, RGS4 and GAIP bind tightly to and exhibit GAP activity towards G(alpha) i, G(alpha) o and G(alpha) t, but not G(alpha) s. RGS10 increases the GTP hydrolytic activity of several members of the G(alpha) i sub-family, including G(alpha) i-3, G(alpha) z and G(alpha) o.