Since specific benzodiazepine (Bz) binding sites have been found in the vision and oculomotor control areas of the central nervous system (CNS), the fast phases of optokinetic nystagmus (OKN) should be affected by Bz administration. In this study, we examine the effects of Bzs on OKN fast phases under closed- and open-loop experimental conditions. Six normal subjects participated in the experiments. The eye movements were measured by the magnetic field, search coil technique, 90 min after diazepam or placebo administration. The study was performed in a randomized, double-blind fashion. After diazepam, the mean amplitude (MAmp) and mean peak velocity (MVel) of OKN fast phases decreased significantly under both experimental conditions. The percentage decreases in MAmp and MVel under the open-loop condition were significantly larger than those under the closed-loop condition. The results indicate that the fast phases of OKN could sensitively reflect the pharmacodynamic effects of Bzs on the CNS.