Eighteen healthy volunteers of both sexes, aged 20-35 years, were tested in the morning after three nights of medication with brotizolam 0.25 mg, nitrazepam 5 mg or placebo on a monotonous simulated driving task. The effect measures were subsidiary auditory reaction time and time outside road. Measurements of self-rated alertness were carried out as well. No effects were demonstrated from treatments on either measure. Nitrazepam however tended to score worst on all measures, except time outside road which could not be analysed with respect to statistical significance because of an insufficient number of subjects leaving the road. Twelve of the subjects were also tested immediately after drug intake on the first night of each medication period. Reaction time decrement was observed in both active drugs conditions with no difference between the two. The other measures, however non-significant, pointed in the same direction with the greatest decrement for nitrazepam.