In this article, a general nonlinear simultaneous equations framework for the econometric analysis of models of intervention in foreign exchange markets by central banks in response to deviations of exchange rates from target levels is proposed. The instrumental variables estimation of possibly nonlinear response functions and tests of intervention, when the functional form may be nonlinear, asymmetric, and may contain unknown shape parameters, is considered. The methodology applies techniques developed for testing in the presence of nuisance parameters unidentified under a null hypothesis to a nonlinear simultaneous equations model. The results of an empirical analysis of stabilization activity of the Bank of Canada, for the period from 1953 to 2009, with regard to the Canada–U.S exchange rate are reported here. A nonlinear specification is found necessary to capture activity after 1998.