1 In a double-blind, placebo controlled study, the efficacy of Ro 15-1788, a new benzodiazepine antagonist, in blocking the cognitive, psychomotor and subjective effects of diazepam, was investigated in a group of six healthy male volunteers. 2 The central effects of orally administered diazepam (40 mg) were most pronounced 1 h after dosing and persisted for 9 h with decreasing severity. 3 Concurrent oral administration of Ro 15-1788 (200 mg) completely prevented the impairment in cognitive and psychomotor function observed after diazepam alone. 4 The duration of action of Ro 15-1788 was shorter than that of diazepam. 5 Plasma diazepam levels after administration of the diazepam/antagonist combination were very similar to those observed following diazepam alone.