Marihuana is known to affect aspects of behaviour important to driving performance. Its increasing use, both alone and with alcohol, make it a potential traffic safety problem. The effects of marihuana, alone and in combination with alcohol, were investigated using a driving simulator with realistic car dynamics. Thirty-five subjects were each tested at three dose levels of marihuana. The subjects were divided into three groups with each group receiving one of three alcohol doses in combination with each of three marihuana doses. Marihuana was found to significantly impair drivers' ability to control speed and lane position, and to affect perceptual functions involved in driving. The alcohol treatment had few significant effects, possibly due to between group differences exacerbated by the experimental design.