Heterotrimeric GTP-binding proteins (G proteins) regulate cellular activity by coupling to hormone or sensory receptors. Stimulated receptors catalyse the release of GDP from G protein alpha-subunits and GTP bound to the empty alpha-subunits provides signals that control effectors such as adenylyl cyclases, phosphodiesterases, phospholipases and ion channels. Three cytoplasmic loops of the activated receptor are thought to interact with three sites on the heterotrimeric G protein to provide high-affinity interaction and catalyse G-protein activation. The carboxyl terminus of the alpha-subunit is particularly important for interaction with the receptor. Here we study the structure of part of the active interface between the photon receptor rhodopsin and the G protein transducin, or Gt, using nuclear magnetic resonance. An 11-amino-acid peptide from the C terminus of the alpha-subunit of Gt (alpha t (340-350)) binds to rhodopsin and mimics the G protein in stabilizing its active form, metarhodopsin II. The peptide alpha t (340-350) binds to both excited and unexcited rhodopsin and conformational differences between the two bound forms suggest a mechanism for activation of G proteins by agonist-stimulated receptors. Insight into receptor-catalysed GDP release will have broad application because the GTP/GDP exchange and the intrinsic GTPase activity of GTP-binding proteins constitute a widespread regulatory mechanism.