Abstract Fifteen healthy volunteers aged 18–35 years took part in this three period crossover study evaluating a portable performance tester designed for roadside use. They received by mouth placebo and two doses of ethanol on separate days. Doses were calculated to produce blood alcohol levels of 50 and 80 mg/100 ml. Testing was carried out before the drink and starting at 40 min after the drink. Breathalyser readings showed peak blood alcohol levels of 54.4 mg/100 ml (S.D 11.1) for the smaller dose and 83.0 mg/100 ml (S.D. 8.4) at the larger dose. Significant impairment was seen with the larger dose of ethanol. Response time was increased for the arrow flankers test (attention in the presence of distractors), and errors were increased for paired associates (visuospatial working memory) and for length estimation (judgement). A composite measure showed a clear dose-related pattern of impairment. These results indicate that a short test battery taking about ten minutes to complete can reliably show the effects of ethanol under controlled laboratory conditions. Further work is needed in the field, and with a more varied population to assess the use of such a device to assess impairment due to alcohol and drugs at the roadside.