Hourly measurements of bile acids, bacterial flora, and pH were made from gastric aspirates collected over 24 hours in subjects taking a controlled diet. Significantly higher concentrations of total bile acids were found following truncal vagotomy and antrectomy (TV+A), 2.73±1.16 SEM mmol/l, than after truncal vagotomy and pyloroplasty (TV+P), 0.88±0.56 mmol/l (p<0.002), and controls, 0.43±0.22 mmol/l (p<0.002). These observations were due to differences in nocturnal reflux only. Free secondary bile acids were identified more frequently after TV+A (19% of aspirates) than TV+P (6%) or controls (2%).Streptococcus faecalis andVeillonella spp, the principal organisms known to deconjugate and hydrolyze bile acids, were present in 65% of aspirates after TV+A compared with 35% after TV+P, and 33% of the controls. Mean pH values were 5.12 after TV+A, 2.82 after TV+P, and 2.14 in the controls. We conclude that antrectomy is associated with a greater incidence of duodenal reflux and a more suitable environment for the formation of secondary bile acids than vagotomy and pyloroplasty or in normal subjects.