Subjects were selected from male applicants who had at least 3 years driving experience, were 21-45 years old, weighed 135-200 pounds, had at least 20/30 vision, and were moderate to light-heavy users of alcohol. For the marijuana study, selected subjects used marijuana at least weekly and at most four times weekly. Fifteen subjects were tested in the secobarbital study. In the marijuana-alcohol and diazepam-alcohol studies, 45 subjects were divided into 3 groups, each group receiving 1 of the 3 doses of alcohol along with their drug doses. Each subject was tested on three occasions in the simulator at each of three dose levels in a repeated-measures design. The simulator tested seven driving tasks requiring perceptual and perceptual-motor abilities. All drugs impaired performance on a wide range of simulated driving tasks. Tabular and graphic data and 32 references.