The effects of diazepam, 5 mg t. i. d., or lithium administered for two weeks on psychomotor skills was examined in 20 healthy male volunteers. The drugs were administered either with alcohol 0.5 g/kg or a placebo. Psychomotor skills were measured by a choice reaction test, two co-ordination tests and an attention test. Serum drug concentrations and blood alcohol levels were also measured. Alcohol impaired all the psychomotor factors tested. Diazepam improved choice reaction performance and slightly eye-hand co-ordination. Lithium impaired the choice reaction performane. Alcohol and diazepam potentiated each other's harmfull effects, whilst lithium tended to antagonize the effects of alcohol, except on co-ordination. The combination of alcohol 0,5 g/kg and lithium may be dangerous in motor vehicle drivers. Alcohol and diazepam together also had very deleterious effect on psychomotor skills related to driving.