GAO reviewed the implementation of the North American Free Trade Agreement's provisions on cross-border commercial truck traffic, focusing on: (1) efforts to make the differing U.S. and Mexican trucking regulations compatible; (2) the major differences in U.S. and Mexican regulations and operating and enforcement practices that adversely affect highway safety and infrastructure; and (3) federal and state governments' readiness to ensure that Mexican trucks comply with U.S. trucking regulations. GAO found that: (1) the United States and Mexico have made some progress in developing compatible trucking standards, but standardization of some regulations, such as vehicle size and weight, may never occur; (2) there are major differences between U.S. and Mexican trucking regulations and operating and enforcement practices that could adversely impact highway safety and infrastructure; (3) negotiators have standardized procedures on drivers' age, inspection criteria, traffic control devices and road signs, and certain hazardous materials, but further standardization of trucking regulations is not scheduled for completion until 1997; (4) where compatibility cannot be reached, foreign trucks must comply with the host country's regulations, but enforcement efforts will be the key to compliance; (5) Mexico is establishing a truck safety enforcement program, but it lacks personnel and other resources to implement it; (6) most Mexican trucks entering the United States do not meet U.S. safety standards; (7) the four U.S. border states' enforcement readiness varies significantly; (8) the Customs Service is permitting Texas to set up truck inspection capabilities within its facilities because it faces the greatest enforcement burden and has relatively limited resources; and (9) in addition to northbound traffic, southbound and east-west traffic will complicate the states' enforcement burden.