Objectives: The barostat is the gold standard for measurement of proximal gastric accommodation. Ultrasonography can be used to measure gastric volume. Aim was to investigate effects of the barostat bag on gastric accommodation and transpyloric flow. Methods: Accommodation after a liquid meal (300 ml, 450 kcal) was measured twice at random in 8 healthy volunteers. Proximal accommodation was measured once using barostat (BS) and once using ultrasound (US). Antrum accommodation was measured using US. Bag volume (BV), antral area (AA), proximal gastric area (PGA) and proximal gastric diameter (PGD) data were assessed before and 1, 5, 15, 30, 40, 50 and 60 min postprandially. Transpyloric flow was measured using Doppler 1-5 minutes postprandially. Results: Fasted, AA size was not affected by the barostat bag (1mmHg>MDP) (2.7(0.5) vs. 2.6(0.3) cm²). Postprandially AA's were larger with the bag present (ANOVA, p<0.04). Maximum AA was reached with the bag 5 min, without 1 min postprandially (15.1±2.3 vs. 9.4±1.5 cm², p<0.03). Furthermore AA's were related to BV's (r=0.57, p<0.01). After bag deflation, AA decreased (11.9±1.8 to 7.0±0.9 cm², p=0.02) and was comparable to the 60 min AA size without the bag (7.1±1.2 cm², p=0.76) present. Proximal gastric radius calculated from the BV's and PGD's was larger with the bag present (ANOVA, p<0.001). No effect on early gastric emptying was observed. Conclusions: Postprandial the barostat bag causes dilatation of the antrum, due to meal displacement without influencing early gastric emptying. This antral dilatation is likely to induce exaggerated proximal gastric relaxation observed in studies using the barostat to evaluate fundic accommodation.