This paper examines the determinants of the voting behavior of the U.S. House of Representatives on NAFTA and GATT. Public choice theory suggests that the voting behavior of legislators is influenced by the interests of their constituencies, special interest politics, and by their ideology. The paper uses probit analysis to test the significance of the above factors. The results indicate that the constituents’ economic interests were significant determinants of the House of Representatives voting on NAFTA and, to a lesser degree, GATT.