Motion illness (MI) is a common aeromedical problem that may occur in pilots exposed to increased gravitational (G) forces in the head-to-foot direction (+Gz). Since MI may affect autonomic nervous functions including cardiovascular control we hypothesised that it might interfere with cardiovascular responses to high G-forces, thereby decreasing G-tolerance. G-tolerance and cardiovascular responses to increased +Gz-load were studied in 9 subjects in a centrifuge in two conditions. In the MI condition the subject was exposed to a Motion Illness Provocative Stimulation (MIPS) comprising repeated rapid changes in G-load in combination with a regimen of head movements. In the Control condition (C) the subject was exposed to the same cumulative G-stress, but without the MIPS. Mean Arterial Pressure (MAP) was measured. An index of peripheral vascular resistance was achieved by measuring the difference in skin temperature between the forearm and fingertip Dforearm-fingertip. MIPS decreased (P<0.01) Gradual-Onset-Rate (GOR) G-tolerance from 5.1 ± 1.0 G (mean±SD) to 4.6±0.9 G. There was no change in GOR-tolerance in the C condition. Also, Rapid-Onset-Rate G-tolerance was lower (P<0.001) in the MI (2.9±0.5 G) than in the C (3.4±0.3 G) condition. MIPS reduced MAP by 10-12 mmH and Dforearm-fingertip by -4.2±4.1 °C (P<0.05). In the C condition MAP and Dforearm-fingertip were unaffected. Thus, motion illness may reduce the tone of peripheral resistance vessels to the extent that it results in a drop in arterial pressure large enough to significantly diminish the capacity of an individual to withstand increased +Gz-forces. This may have implications for pilots flying high-performance aircraft.