Horizontal and vertical optokinetic nystagmus (OKN) was studied in 20 healthy adults. Although the horizontal OKN showed no directional difference, a statistically significant difference was found between horizontal and vertical OKN. Upward optokinetic pursuit was better on average than downward pursuit, but more variable. The inferiority of vertical OKN seems to indicate a suppression of optokinetic pursuit due to a different direction of the rotational axis from that of gravity. Regarding the vertical OKN findings, it is speculated that manifest directionality found in quadrupeds is modified in men by the change of their visual field on forward locomotion accomplished by their upright walking posture.