On April 1, 1994, Ontario, Canada, instituted a new graduated driver license (GDL) system that effectively set the legal blood alcohol content (BAC) threshold at zero for the first few years of a youth's driving eligibility. I use data from the 1983-2001 Ontario Student Drug Use Surveys (OSDUS) to examine whether the Zero Tolerance (ZT) policy reduced self-reported drinking and alcohol-involved driving among youth. I find that rates of drunk driving reported by 16- to 17-year-olds--who faced new, lower legal limits after adoption of the ZT policy--were about 5 percentage points lower after the law was implemented. Visual inspection of the data, however, shows that the estimated reduction is an artifact of a pre-existing trend: Drunk driving rates in this age group were falling steadily throughout the 1980s and into the 1990s. Estimates that account for this pre-existing trend or that consider shorter windows around the 1994 implementation date return effects on alcohol-involved driving that are either small and statistically insignificant or large and implausibly signed (positive). These null findings are robust to using the associated change in outcomes for slightly younger (14-15) or slightly older (19-20) youths as controls in a difference-in-differences framework. I similarly find no robust effect on drinking participation. This suggests that Ontario's age-targeted drunk driving law--despite being harsher than similar policies in the United States--was not responsible for reductions in Canadian youth road fatalities over the past two decades.