Abstract
An experimental procedure was used to evaluate the effects of marihuana smoking and various levels of blood alcohol upon several components of the driving task. It was found that performance decrement increased with blood alcohol content, that differences in performance found under alcohol were in evidence among the same subjects in non-alcohol trials, that marihuana produced performance decrement to a much lesser degree than alcohol, and that experimenters could predict the driving performance of drinking subjects by observation of their behaviour just prior to the experimental run.