Abstract： Objective To observe changes in free amino acid concentrations in the serum of humans following exposure to acceleration and to provide clues to the etiology and pathogenesis of motion sickness Methods Sixty human volunteers were exposed to repetitive acceleration stimulation by using 6 - degree - offreedom ship motion simulator. Following termination of the acceleration exposure, the 60 subjects were divided into two groups. Subjects who developed nausea and vomiting were classified as the motion sickness group, and those without the above symptoms were classified as the non - motion sickness group. Then, serum levels of free amino acids in the motion sickness and non - motion sickness groups were determined by High Performance Liquid Chromatography (HPLC) both before and after exposure and a comparison was made between them.Results ( 1 ) Serum aspartic acid and glutamic acid concentrations in all the subjects increased markedly,whereas serum serine, threonine, tyrosine, methionine, phenylalanine, leucine and lysine concentrations in all the subjects decreased following exposure to acceleration. (2) Before exposure, serum glutamic acid,isoleucine and cystine concentrations in the subjects of the non - motion sickness group were obviously higher than those of the motion sickness group, while phenylalanine and leucine concentrations in the motion sickness group elevated significantly following exposure, when a comparison was made between them. (3) After exposure, serum glutamic acid concentrations in the subjects of both groups all increased, while serine and lysine concentrations in both groups decreased. Furthermore, significant differences in the amplitude of glutamic acid increase rate could be seen between the motion sickness group and the non - motion sickness group,following exposure. (4) Serum concentrations of tyrosine, cystine, methionine, phenylalanine, isoleucine and leucine decreased considerably in the non - motion sickness group, whereas no obvious changes could be noted in the motion sickness group. Conclusions Acceleration exposure could induce changes in certain amino acid concentrations. Significant differences in the change rate of certain serum amino acid concentrations could be seen between the motion sickness and non - motion sickness groups. Low concentrations of glutamic acid,isoleucine and cystine might be correlated to the sensitivity of motion sickness while glutamic acid concentrations elevated significantly following acceleration exposure. Changes in serum phenylalanine and leucine concentrations might be related to the mechanism of motion sickness.