To evaluate the antisecretory activity of berberine sulfate (BS), we studied 165 adult patients with acute diarrhea due to enterotoxigenic Escherichia coli (ETEC) and Vibrio cholerae in randomized controlled trials. In patients with ETEC diarrhea who received 400 mg of BS in a single oral dose, the mean stool volumes were significantly less than those of the controls during three consecutive 8-hr periods after treatment (P less than .05). At 24 hr after treatment, significantly more patients who were treated with BS and had ETEC diarrhea stopped having diarrhea as compared with the controls (42% vs 20%, P less than .05). In patients with cholera who received 400 mg of BS, the mean 8-hr stool volume during the second 8-hr period after treatment declined to 2.22 liters, which was significantly less than the 2.79 liters found in the controls (P less than .05). However, patients with cholera who received 1200 mg of BS plus tetracycline did not have significant reduction in stool output compared with patients who received tetracycline alone. No side effects of BS were noted. These results indicated that BS is an effective and safe antisecretory drug for ETEC diarrhea, whereas the activity against cholera is slight and not additive with tetracycline.