Because it is of the same taxonomic order as man, the squirrel monkey has been introduced as a nonhuman primate model for the study of such aspects of motion and space sickness as susceptibility, prevention, treatment, and neural mechanisms. In this study on susceptibility, the specific aims were to examine the effects of combined vertical rotation and horizontal acceleration, phenotype, sex, visual cues, morning and afternoon testing, and repeated test exposures on incidence, frequency, and latency of emetic responses. The highest emetic incidence of 89%, with an emetic frequency of 2.0, during 60 min, and a latency of 19 min from onset of testing, occurred at 25 rpm and 0.5 Hz linear acceleration. Susceptibility--defined by incidence, frequency, and/or latency of emesis--was significantly higher in Bolivian than Colombian phenotypes, in the presence of visual cues, in males, but not at different periods of the day or with two successive test exposures. Since the emetic responses were quite similar to man in the eliciting motion stimuli, it was concluded that the squirrel monkey represents a very suitable primate model for studies of motion and space sickness.