G protein-coupled receptors (GPCRs) are versatile chemical sensors, which transmit the signal of an extracellular binding event across the plasma membrane to the intracellular side. This function is achieved via the modulation of highly dynamical equilibria of various conformational receptor states. Here we have probed the effect of pressure on the conformational equilibria of a functional thermostabilized beta(1)-adrenergic GPCR (beta(1)AR) by solution NMR. High pressure induces a large shift in the conformational equilibrium (midpoint similar to 600 bar) from the preactive conformation of agonist-bound beta(1)AR to the fully active conformation, which under normal pressure is only populated when a G protein or a G protein-mimicking nanobody (Nb) binds to the intracellular side of the beta(1)AR center dot agonist complex. No such large effects are observed for an antagonist-bound beta(1)AR or the ternary beta(1)AR center dot agonist center dot Nb80 complex. The detected structural changes of agonist-bound beta(1)AR around the orthosteric ligand binding pocket indicate that the fully active receptor occupies an similar to 100 angstrom(3) smaller volume than that of its preactive form. Most likely, this volume reduction is caused by the compression of empty (nonhydrated) cavities in the ligand binding pocket and the center of the receptor, which increases the ligand receptor interactions and explains the similar to 100-fold affinity increase of agonists in the presence of G protein. The finding that isotropic pressure induces a directed motion from the preactive to the fully active GPCR conformation provides evidence of the high mechanical robustness of this important functional switch.