The benzodiazepines are typified by a profile of side effects which includes drowsiness, ataxia and incoordination. Ro 15-1788, an imidazodiazepine derivative, exhibits marked antagonism of the behavioural and biochemical effects of the benzodiazepines in animals and man. It is devoid of any behavioural activity in animals, except at very high doses. In the present study the effects of single rising oral doses of Ro 15-1788 on cognitive, psychomotor and subjective function in man have been assessed using a battery of psychometric tests designed to identify the sedative action of the benzodiazepines. At all doses up to 600 mg, Ro 15-1788 demonstrated none of the classical behavioural effects of the benzodiazepines.