Cardiopulmonary bypass (CPB) is known to impair the integrity of the gastrointestinal tract. However, little is known about the movement behavior of the gastrointestinal tract during CPB. This study was aimed to assess the gastrointestinal motility with sonomicrometry, a distance measurement using ultrasound, in a porcine model of CPB. Twelve pigs weighing 70-112 kg were having a standard hypothermic CPB for 120 min either with the nonpulsatile flow (n = 6) or the pulsatile flow (n = 6). Before CPB, piezoelectric echo crystals were placed either along the longitudinal or the circular axis of the pylorus. Patterns of gut movement and the total sonomicrometric activity (TSA) were recorded at several time intervals during experiments as qualitative and quantitative parameters of gut motility. Results showed that the intact regular rhythmic pattern of gut movement was detected before CPB. This pattern changed little when CPB started, but it disappeared at 60 min when the body temperature lowered down to 32 degrees Celsius. During the same period, the TSA reduced significantly along the longitudinal as well as the circular directions of the pylorus. There was no significant difference between the nonpulsatile and pulsatile groups. Gut blood flow reduced significantly in both groups, but it was not associated with the reduced sonomicrometric activity. In conclusion, gastrointestinal motility during CPB can be measured qualitatively and quantitatively by sonomicrometry in a large animal model. Suppression of gut motility during CPB does not seem to be associated with the mode of perfusion but with the reduced body temperature during the hypothermic phase of CPB.