This paper examines the determinants of the voting behavior of the U.S. Senate on NAFTA and GATT. Public choice theory suggests that the voting behavior of senators 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 and special interest money were significant determinants of the Senate voting on NAFTA but not on GATT.