This was the first study that measured the drugs' combined effects in a natural setting, i.e., on traveled roads in normal traffic. Study participants were 18 persons between the ages of 20 and 28 who were licensed to drive an automobile and who smoked marijuana and drank alcohol at least once a month. Subjects were administered drugs and a placebo according to a balanced, six-way, observer- and subject-blind, cross-over design. On separate evenings they were given weight-calibrated doses of THC and alcohol, or placebos for one or both substances as follows: alcohol placebo plus THC placebo; alcohol placebo plus THC 100 :g/kg; alcohol placebo plus THC 200 :g/kg; alcohol plus THC placebo; alcohol plus THC 100 :g/kg; and alcohol plus THC 200 :g/kg. The initial alcohol dose was sufficient to produce a peak blood alcohol concentration (BAC) of approximately 0.07 g/dl. Booster doses were later given to sustain BAC around 0.04 g/dl during testing. On each occasion, 30 minutes after smoking the sample given, subjects performed four driving tests in the evening hours after dark between 21:00 and 23:15. These involved two repetitions of a Road Tracking Test and two repetitions of a Car Following Test. The former test measured the subjects' ability to maintain a constant speed of 100 km/h (62 mph) and a steady lateral position between the delineated boundaries of the right (slower) traffic lane. The latter test measured the subjects' reaction times and headway variability while driving 164 feet behind a vehicle that executed a series of alternating acceleration and deceleration maneuvers. The study found that THC doses (marijuana) alone and alcohol doses alone significantly impaired the subjects' Road Tracking and Car Following performances. The magnitude of the mean effects were minor after alcohol and THC 100 :g/kg and moderate after THC 200 :g/kg. Both THC doses in combination with alcohol severely impaired the subjects' performance in both tests. Although the effects of THC alone in doses up to 200 :g/kg might be categorized as "moderate," they became "severe" when THC was combined with a moderate dose of alcohol. 5 figures, 7 tables, and 33 references