The aim of the present study was to examine the effects of acute exposure to hypobaric hypoxia on gastric and colonic motilities. Wistar rats, which were instrumented chronically with strain gauge force transducer to measure gastric and colonic motilities, were exposed acutely to hypobaric hypoxia [0.5 atmosphere absolute (ATA, 380 Torr)] over 1 hr. In a separate group, the gastric branches of the vagal nerves were cut and underwent the same experimental protocol. Each contraction wave of the stomach and colon was analyzed into frequency and area under the curves which were then averaged every 10 min. Acute exposure to 0.5 ATA resulted in significant (P<0.05) decreases in frequency and area of gastric contraction wave by 0.5 ± 0.1 cycles per min and 64.6 ± 4.0%, respectively. Gastric vagotomy abolished completely the suppression in the area observed in the intact rats during the 0.5 ATA exposures. Colonic motility increased significantly only at the start and end of exposure to 0.5 ATA and sham exposure [1 ATA (760 Torr), time control] in both intact and vagotomized rats. These data suggest that the acute suppression of the area of the gastric contraction wave occurred during 0.5 ATA exposure is likely to be mediated by the vagal nerve.