G protein-coupled receptors (GPCRs) are a family of seven transmembrane helical proteins that initiate a cellular response to an environmental signal. Once activated by an extracellular signal, GPCRs trigger the intracellular signal transduction cascade by activating a heterotrimeric G protein. The interaction between the G protein and the receptor, which triggers the signal transduction, is the focus of intense interest. Three-dimensional structures of the ground state of only one GPCR, rhodopsin, are currently available, but since the G protein cannot bind to this structure, these structures did not lead to an understanding of the activation process. The recent publication of an excited state structure for the same GPCR (and comparison to the ground state structures), in conjunction with other recent biochemical data, provides new insight into G protein activation. We find that the structure data and the biochemical data, for the first time, point to a specific mode of interaction between the G protein and the receptor. Furthermore, we find that transducin (G(t)) must alter its conformation to bind to the activated receptor; the "lock and key" fit heretofore expected is likely not the correct model. We suggest that a conformational distortion, driven by the energy of binding, is induced in G(t) when it binds to the activated receptor. The conformational change in turn enables the exchange of GTP for GDP and the dissociation of the subunits. This is an example of "induced fit" originally proposed by Koshland to describe enzyme-substrate interactions.