It has been suggested that an increasing number of people drive after taking diazepam, and while the clinical use of anxiolytics is expected to improve the patients' everyday level of performance, experimental studies of the effects of diazepam on driving have variously indicated performance decrements, no effects, or performance increments. Possible reasons for this inconsistency are variation between and within subjects, and differential effects of the drug on the various components of task performance. These factors were investigated by an experiment designed to compare the effects of a single 10 mg oral dose of diazepam between and within groups and between and within individuals, and to examine effects of the drug on separate behavioural components of a driving task. Twenty drivers took part in the study, which assessed their ability, their degree of confidence in their ability, and their willingness to drive through various sizes of gap. The drivers were divided into two groups of ten, and were tested individually on each of 2 days. Group 1's treatment was placebo-diazepam, and group 2's was placebo-placebo. When analysed by group, the results showed a significant increase (P<0.01) in="" group="" 2's="" willingness,="" and="" significant="" decrease="">P<0.001) in="" group="" 1's="" willingness="" to="" attempt="" gaps,="" which="" suggests="" greater="" caution="" under="" diazepam="" treatment.="" there="" was="" no="" change="" in="" either="" group's="" confidence,="" but="" group="" 1="" showed="" a="" significant="" increase="">P<0.05) in="" their="" ability="" variance.="" when="" analysed="" by="" individual,="" a="" wide="" variety="" of="" significant="" changes="">P<0.05 or="" better)="" were="" found="" in="" ability="" and="" confidence="" for="" group="" 1="" individuals,="" some="" changing="" for="" better,="" and="" some="" for="" worse,="" under="" diazepam="" treatment.="" no="" significant="" changes="" in="" ability="" or="" confidence="" were="" found="" for="" group="" 2="" individuals.="" all="" group="" 1="" individuals="" were="" less="" willing="" to="" attempt="" gaps="" under="" diazepam="" treatment,="" while="" all="" group="" 2="" individuals="" were="" more="" willing="" to="" attempt="" gaps="" under="" continued="" placebo="" treatment.="" the="" validity="" of="" these="" findings="" is="" discussed,="" in="" terms="" of="" real-life="" driver="" behaviour,="" and="" it="" is="" concluded="" that="" there="" may="" be="" a="" wide="" inter-individual="" variation="" in="" the="" effects="" of="" diazepam="" not="" necessarily="" apparent="" in="" grouped="" data,="" and="" that="" diazepam="" may="" have="" differential="" effects="" on="" the="" decision-making="" and="" perceptual-motor="" components="" of="" driver="" behaviour="" in="" a="" gap-judging="">