Because of alterations in bile acid metabolism known to accompany surgical vagotomy, we assessed the effect of medical cholinergic blockade on bile acid metabolism. Five normal volunteers were studied by standard isotope dilution techniques in a control period and again after 2-3 wk of propantheline bromide (Pro-Banthine) 15 mg four times each day. Cholinergic blockade increased the mean cholic acid pool from 1230 μmol to 1860 μmol (P < 0.005) and increased mean chenodeoxycholic acid pool from 1760 μmol to 2180 μmol (P < 0.005). Mean tool bile acid pool increased from 4100 μmol in the control period to 5590 μmol on Pro-Banthine (P < 0.025). Pool size of the secondary bile acid, deoxycholic acid, did not change during Pro-Banthine treatment. As in the case of surgical vagotomy, these changes in bile acid pools were largely a result of reduction in fractional removal rate which for cholic acid averaged 0.714 day-1 in the control period and 0.451 day-1 during Pro-Banthine (P < 0.05). For chenodeoxycholic acid mean fractional removal rate in the control period was 0.411 day-1 and in the Pro-Banthine period was 0.278 day-1 (P < 0.05). For both bile acids, synthesis was not significantly altered by Pro-Banthine. Part of the mechanism of these changes may have been prolonged small bowel transit which averaged 39 min in the control period and 94 min during Pro-Banthine (P < 0.005). Gallbladder emptying rate slowed from a control mean of 0.027 min-1 to 0.020 min-1 during Pro-Banthine, but this change was not statistically significant. Gallbladder bile lipid composition was not appreciably affected by Pro-Banthine administration.