G-PROTEIN-COUPLED receptors are thought to have an inactive conformation (R), requiring an agonist-induced conformational change for receptor/G-protein coupling(1-3). But new evidence suggests a two-state model(4-19) in which receptors are in equilibrium between the inactive conformation (R), and a spontaneously active conformation (R*) that can couple to G protein in the absence of ligand (Fig, 1). Classic agonists have a high affinity for R* and increase the concentration of R*, whereas inverse agonists have a high affinity for R and decrease the concentration of R*. Neutral competitive antagonists have equal affinity for R and R* and do not displace the equilibrium, but can competitively antagonize the effects both of agonists and of inverse agonists. The lack of suitable in vivo model systems has restricted the evidence for the existence of inverse agonists to computer simulations(7,8) and in vitro systems(5,9-12,20-23). We have used a transgenic mouse model in which there is such marked myocardial overexpression of beta(2)-adrenoceptors that a significant population of spontaneously activated receptor (R*) is present, inducing a maximal response without agonist(24). We show that the beta(2)-adrenoceptor ligand ICI-118,551 functions as an inverse agonist, providing evidence supporting the existence of inverse agonists and validating the two-state model of G-protein-coupled receptor activation.