The effect of inhaling nitrous oxide on pursuit eye movements (PEM) and voluntary saccades was studied in nine healthy subjects. Eye movements were recorded before, during, and 10 min after exposure for 15 min to normoxic mixtures of 14%, 21%, and 28% nitrous oxide. At all concentrations, there was a significant decrease in gain of PEM at a target velocity of 60 degrees/s, the decrease being of comparable magnitude at concentrations of 14% and 21%. By 10 min after exposure to nitrous oxide, the gain of PEM had recovered to pre-test values. There was also a highly significant decrease in the peak velocity of voluntary saccades at all concentrations of nitrous oxide; the decrease was more pronounced at the higher concentrations, and the existence of a dose-dependent relationship is suspected. Exposure to 21% nitrous oxide for only 2 min was enough to induce significant reduction in peak velocity of voluntary saccades. By 10 min after exposure to nitrous oxide, the peak velocity of voluntary saccades had recovered but had not reached pre-test values. The findings suggest that both PEM and voluntary saccades decrease with reduced alertness in mild narcosis. Moreover, as the effects of nitrous oxide are considered typical of inert gases in general, our findings suggest that voluntary eye movements may also be a suitable variable for use in assessing the effects of inert gas narcosis.