The effect of diazepam on spatial visualization was studied using a double-blind crossover design. Diazepam 10 mg (or placebo) was given orally to 12 healthy young men at bedtime, followed again by 10 mg (or placebo) the next morning. One hour after the administration of the morning dose, the subjects performed a mental rotation task. Diazepam effects were analyzed in terms of an information processing model previously constructed to account for performance of the task. Using this method, the slope and intercept parameters of a linear function relating reaction time to task condition (i.e., angular difference in orientation between two identical stimulus figures) were computed for diazepam and placebo conditions. Diazepam significantly increased the slope and intercept of the mental rotation function (P less than 0.05). These results suggest that diazepam impairs spatial visualization ability.