This paper reports on the enforcement of a zero tolerance (ZT) law implemented on January 1, 1994, in California. Some provisions in that legislation merely intended to make enforcement easy;​ for example, using handheld, portable, breath-testing devices at the road-side. The research employs a model of the expected official actions, target group reactions, and underage fatal crashes to trace the impact of the law on underage drinking and driving. The results indicate little increase in the intensity of enforcement and no change in the perceived risk of arrest among the target group;​ however, results do indicate a possible reduction in drinking drivers involved in fatal crashes in that group.