DIAZEPAM (‘Valium’, Roche) is a recently introduced anti-anxiety drug of the 1,4-benzodiazepine series1,2. Clinically, it is usually said to act for 6–8 h, but quantitative data about the time course of functional changes in man do not appear to be available. Measurements of changes in the visual critical flicker fusion threshold (CFFT) and its auditory equivalent, the auditory flutter fusion threshold (AFFT), have been used for this purpose and the results are reported below. The fusion thresholds measure the ability of subjects to discriminate between rapidly repeated sensory stimuli, and both have been shown to be sensitive to the effects of small doses of centrally acting drugs3–6, which may impair or improve this discrimination.