An open-label, parallel comparison of loperamide hydrochloride (Imodium A-D) and bismuth subsalicylate (Pepto-Bismol) was conducted using nonprescription dosages in adult students with acute diarrhea (three or more unformed stools in the preceding 24 hours plus at least one additional symptom of enteric infection). For the two-day study period, the daily dosage was limited to 8 mg (40 ml) for loperamide-treated subjects and to 4.9 g for bismuth subsalicylate-treated subjects. At these dosages, loperamide significantly reduced the average number of unformed bowel movements relative to bismuth subsalicylate. Following the initial dose of treatment, control of diarrhea was maintained significantly longer with loperamide than with bismuth subsalicylate. Time to last unformed stool was significantly shorter with loperamide than with bismuth subsalicylate. In providing overall subjective relief, subjects rated loperamide significantly better than bismuth subsalicylate at the end of the 24 hours. Both treatments were well tolerated, and none of the minor adverse effects reported resulted in discontinuation of therapy. It was concluded that loperamide is effective at a daily dosage limit of 8 mg (40 ml) for the treatment of acute nonspecific diarrhea and provides faster, more effective relief than bismuth subsalicylate.