Bombesin and structurally related peptides including gastrin releasing peptide (GRP) are potent mitogens for Swiss 3T3 cells. The early cellular and molecular responses elicited by bombesin and structurally related peptides have been elucidated in detail. Further understanding of the molecular basis of the potent mitogenic response initiated by bombesin is required in order to elucidate the mechanism by which the occupied receptor communicates with effector molecules in the cell. Transmembrane signalling mechanisms involving either a tyrosine kinase or a guanine nucleotide-binding regulatory protein (G protein) have been proposed. Here we summarize our experimental evidence indicating that a G protein(s) is involved in the coupling of the bombesin receptor to the generation of intracellular signals related to mitogenesis. Evidence for the role of G proteins in bombesin signal transduction pathways has been obtained by assessing the effects of guanine nucleotide analogues on both receptor-mediated responses in permeabilized cells and ligand binding in membrane preparations. We found that [125I]GRP–receptor complexes were solubilized from Swiss 3T3 cell membranes by using the detergents taurodeoxycholate or deoxycholate. Addition of guanosine 5-[γ-thio]triphosphate (GTPγS) to ligand–receptor complexes isolated by gel filtration enhanced the rate of ligand dissociation in a concentration-dependent and nucleotide-specific manner. These results demonstrate the successful solubilization of [125I]GRP–receptor complexes from Swiss 3T3 cell membranes and provide evidence for the physical association between the ligand-receptor complex and a guanine nucleotide binding protein(s).