24 male subjects were divided into 3 groups, on the basis of their susceptibility to motion sickness. All subjects were then required to set a luminous line in an apparently vertical position while viewing the line from a body position which was deviated 70 degrees laterally from the upright. No visible frame of reference was available. A significant relationship between motion sickness susceptibility and errors in judging the vertical was discovered, the "intermediate" susceptibility group making the greatest errors. The role of the vestibular system in visual orientation and motion sickness is discussed. The result also indicates the potential value of using perceptual performance as a tool in the study of motion sickness and its correlates.