The effects of diazepam of several cognitive and performance tasks were investigated in 30 healthy volunteers randomly assigned to three groups: A chronic group received diazepam for 21 days; an acute group received placebo during the same period, except at session 4 when they received diazepam; and a third group received placebo only at the sessions. Diazepam was given orally in a dose of about 0.2 mg/kg. Behavioral sessions were conducted before treatment (practice), after one administration (session 2), after 19 days (session 3), after 20 days (session 4), and 7 days following withdrawal (session 5). A single administration of diazepam produced significant memory impairment in both immediate and delayed free recall. Reduced memory performance was the result of impaired acquisition rather than reduced retention. Comparison of the chronic and acute groups in sessions 3 and 4 and analysis of the performance of the chronic group over sessions indicated the development of some tolerance to the memory impairment with continued administration. No residual memory effects were apparent following withdrawal. No other cognitive or psychomotor functions differed among the three treatment groups.