In order to try to predict effects of environmental tobacco smoke, neurobehavioral effects of mainstream smoke were reviewed and, in conjunction with what is known about body uptake of components of environmental tobacco smoke, conjectures were made about the probable effect of environmental tobacco smoke. Effects of mainstream smoke differ in smokers and nonsmokers. Mainstream smoke has a beneficial effect on vigilance in habitual smokers. The effect in nonsmokers is less clear and may be disruptive. In both smokers and nonsmokers mainstream smoke produces increased tremor and reduced fine motor skills. The neurobehaviorally active substances in mainstream smoke appear to be nicotine and carbon monoxide. It appears that COHb is the more important consequence of environmental tobacco smoke for neurobehavioral effects, since nicotine levels in nonsmokers only reach a small fraction of those in smokers.