The increasing prescription of psychotropic medication in the U.K. and elsewhere is likely to result in a higher proportion of road users driving whilst under the influence of such drugs. It is therefore important to study their effects upon all aspects of the driving task. This paper concerns the effects of four commonly prescribed tranquillizing drugs and small amounts of alcohol upon Kinetic Visual Acuity (K.V.A.) - the ability to perceive a moving object travelling towards the eye at a constant speed in a horizontal plane. Previous research by Professor Suzumura (Nagoya University, Japan) has suggested that K.V.A. is an important driver-related function. Using a fully-randomised double-blind procedure, four groups of 20 subjects (10 men and 10 women) were each tested under the influence of four drugs (Trifluoperazine, chlordiazepoxide, haloperidol, amylobarbitone sodium) and a placebo. A fifth group was given a double-placebo treatment. Within each treatment, the subjects were also tested with and without alcohol (0.5g./kg. body weight). Trifluoperazine produced a significant impairment in K.V.A. values for male subjects and for female subjects under alcohol. Amylobarbitone sodium and chlordiazepoxide produced some improvement in K.V.A. values, whereas haloperidol produced different effects in males and females. Alcohol did not produce a significant overall effect upon K.V.A. values. If significant changes in K.V.A. occur in the real driving situation then a potential danger may exist. It is suggested that physicians should warn patients of the possible danger in driving during the early stages of treatment.