The present study was undertaken to compare the efficacy of a new calmodulin antagonist, zaldaride maleate, with that of placebo or loperamide in persons with traveler's diarrhea. One hundred seventy-nine patients were randomized to receive loperamide (4 mg followed by 2 mg after each unformed stool), zaldaride maleate (20 mg four times per day), or placebo. During the initial 48 hours of therapy, zaldaride maleate decreased the number of unformed stools by 30% and the duration of illness by 23% when compared with placebo. Loperamide was superior to both zaldaride maleate and placebo during the initial hours of treatment. However, after 48 hours of treatment, loperamide and zaldaride maleate were equally efficacious, decreasing by > 50% the number of unformed stools passed in a 24-hour interval (P, not significant), and were both superior when compared with placebo (P < .0001 and P = .0048, respectively). The apparent superiority of loperamide early in the course of therapy appeared to be related to a loading-dose effect and not to any differences in antidiarrheal properties.