The 3 experimental studies used a total of 93 male subjects who had been driving at least 3 years, were 21 to 45 years old, weighed 135-200 pounds, had at least 20/30 vision, and were moderate to heavy alcohol users. The subjects in the marijuana study used the drug 1 to 4 times a week. The subjects received screening via medical and personality evaluations. Each subject was tested at three different dose levels. The blood alcohol content ranged from 0 to 0.11 percent depending on the test. The doses of secobarbital and diazepam were therapeutic doses. The marijuana doses were considered moderate to strong by the study subjects. The driving simulators tested seven tasks: curve following, control during wind gusts, following another car, avoiding an obstacle, passing, recognizing route signs, and following a car changing speeds or heavy wind gusts. The drugs all impaired perceptual-motor skills, perceptual tasks where response time and detection ability were measured, and decisionmaking tasks. Adding alcohol increased the impairment. Figures, tables, and 33 references.