Under a variety of experimental conditions, nonrestrained adult male squirrel monkeys were subjected to continuous rotation in the horizontal plane at 33 rpm. Severity of motion-induced sickness was quantified by measuring latencies of three responses associated with sickness. Per- and postrotational nystagmus was recorded from a subsample of monkeys with permanent recording electrodes implanted in the bony orbits. Incidence of sickness depended on the time limits of stimulation imposed, and it occurred in a maximum of about 90 per cent of test sessions if rotation was continued for two hours. When subjected to ten once-per-day rotation sessions, only a few monkeys showed evidence of developing transient habituation of onset of motion sickness. Mean frequency of emetic episodes, however, declined over the latter half of the rotation series. Restricting visual cues by blindfolding the monkeys prevented most subjects from vomiting.