This paper examines the impact of mandatory Country of Origin Labeling on American imports of Canadian hogs and pork by testing for structural change. Given the uncertainties over the timing of the implementation and reform of COOL, we implement statistical procedures that endogenously test for structural change over multiple time periods. We find evidence that COOL has impacted U.S./Canada feeder and slaughter hog trade flows. In contrast, we found no evidence of structural change for pork trade flows that could be associated with COOL. [EconLit citations: Q17; C12].