The aim of this study was to compare the nauseogenic potential of low-frequency linear motion in the Earth-vertical versus the Earth-horizontal plane, delivered through the same Z-axis of the head and body. Twelve subjects were challenged with linear motion (0.3 Hz, 1.8 ms-2 rms) through the same head and body Z-axis in the Earth-vertical (sitting upright) versus horizontal (lying on the back), while either performing a continuous visual search task or with their eyes closed. Each subject completed the four conditions on a Latin square design with sessions spaced 1 week apart at the same time of day. Vertical motion was clearly more provocative than horizontal motion, and nauseogenicity of motion was exacerbated by a visual search task. Motion sickness impaired performance of the search task. Motion Sickness Susceptibility Questionnaire (MSSQ) scores correlated with individual susceptibility to the motion challenge. Mean sickness ratings for vertical motion showed some correspondence with those predicted by mathematical models of motion sickness dose response relationships.