Constitutively active thrombin receptors were generated while constructing chimeric receptors to identify the structural basis for thrombin receptor agonist specificity. Substitution of eight amino acids from the Xenopus receptor's second extracellular loop (XECL2B) for the cognate sequence in the human thrombin receptor was sufficient to confer robust constitutive activity. Smaller substitutions within the XECL2B site yielded less constitutive activation, and substitution of several unrelated sequences at this site caused no activation. Expression of the XECL2B receptor caused high basal 45Ca efflux in Xenopus oocytes and high basal phosphoinositide hydrolysis and reporter gene induction in COS cells. Of note, a mutant receptor in which all four of the Xenopus thrombin receptor's extracellular segments replaced the cognate human sequences showed much less constitutive activity than XECL2B and preserved responsiveness to agonist. This partial complementation of the XECL2B phenotype by addition of other Xenopus extracellular structures suggests that the XECL2B mutation causes constitutive activation by altering interactions among the human receptor's extracellular domains. Thus, a change in an extracellular loop of a G protein-coupled receptor can transmit information across the cell membrane to cause signaling, perhaps via a conformational change similar to that caused by agonist binding. Indeed, the site of the activating mutation in XECL2B coincides with a putative agonist-docking site, supporting the hypothesis that agonist interactions with the thrombin receptor's extracellular loops contribute to receptor activation.