In order to demonstrate equilibrium ataxia during motion sickness, we examined 12 normal adult volunteers with sharpened Romberg tests and standing on one leg with eyes closed. They underwent the tests before riding and after riding in a car 10 times on course A with straight streches and unidirectional curves, and after riding 10 times on course B with straight streches, S-shaped curves, and right and left turns. The eight subjects who became sick showed a significant decrease in the duration of steady standing after driving on course B. No significant change was noted in the four subjects who did not become sick. The results support the theories that motion sickness, whether it occurs during walking while wearing horizontally reversing goggles or during riding in a car, is evoked by spatial disorientation which produces not only autonomic nervous symptoms but also equilibrium ataxia.