The results reported in Table 2 imply that the 4.5 percentage point decline in Canadian tariffs in the period 1990-1996 is associated with a 24 percentage-point increase in the probability that an average plant will enter the export market in the period. [...] But the positive fixed effect in the 1990s could also be the result of the weak Canadian dollar and the strong U. S. economic performance in the 1990s. [...] When the sum of the two tariff changes is included, the coefficient on the variable has the expected sign and is significant at the 10 percent level. [...] The size of the coefficient implies that the 1.01 percentage points decline in the sum of U. S. and Canadian tariffs in the period 1990-1996 is associated with a 13 percentage-point increase in the probability of exporters becoming more export intensive.11 Larger plants tended to increase their export intensity more than smaller plants, which is consistent with the argument that scale economies ar [...] To examine the difference in the impact of tariff changes on a plant's decision to export between periods 1984- 1990 and 1990-1996, we have included the interaction of tariff changes and the dummy for period 1990-1996.