The purpose of the present studies was to empirically determine the separate and combined effects of 9 tetrahydrocannabinol (THC) and alcohol on actual driving performance. In the first study, eighteen recreational THC users were treated with drugs and placebo according to a balanced, 6- way, observer and subject blind, cross-over design. On separate evenings they were given THC placebo, THC 100 g/kg and THC 200 g/kg with and without alcohol. Alcohol doses were sufficient to sustain blood alcohol concentrations (BACs) of around 0.4 g/dl during testing. Subjects conducted two driving tests at each occasion: i.e. the Road Tracking Test and a Car-Following Test. In the second study 16 recreational users were treated with drugs and placebo according to a balanced, 4-way, cross-over, observer and subject-blind design. On separate evening they were treated with THC placebo and THC 100 g/kg with and without alcohol. As in the first study, alcohol doses were sufficient to sustain BACs of around 0.4 g/dl during testing. Subjects conducted a City Driving Test. Both THC doses alone, and alcohol alone, significantly impaired the subjects Road Tracking and Car-Following performances. Both THC doses in combination with alcohol severely impaired the subjectsperformance in each test. In the City Driving Test, the combination of THC 100 g/kg and alcohol significantly reduced the frequency of visual search for traffic at intersections. While the effects of THC alone in doses up to 200 g/kg might be categorized as moderate , they become severe when THC is combined with a low dose of alcohol.