Although vestibular nystagmus is known to be affected by variations in mental states, little information is available about the effects of drugs on vestibular responses when a) subjects are either alert or relaxed, and b) visual stimuli are available or denied. In this study, 30 men were assigned to d-amphetamine sulphage (10 mg), secobarbital sodium (100 mg), or placebo (no drug) groups. With subjects alert in darkness, the drugs had no differential effect on rotation-induced vestibular nystagmus; when subjects were relaxed there was significantly less nystagmus than in the alert condition, particularly for the seco-barbital group. With vision permitted, d-amphetamine had no statistically different effect on nystagmus from the placebo. However, subjects given secobarbital were unable to use visual fixation effectively to suppress vestibular eye movements and their visual-following ability as measured by optokinetic nystagmus was also suppressed. Control over the mental activity of subjects and assessment of oculomotor-related functions both with and without opportunities for visual fixation are both important in evaluating drug effects.