Abstract: Loperamide is a novel type of antidiarrheal agent. In mice Loperamide was shown to be a potent blocker of 15-methyl-PGF_2α-induced diarrhea and charcoal progression in the intestine. The inhibitory potency of Loperamide was greater than that of morphine and atropine. The results of experiments on isolated smooth muscles showed that Loperamide inhibited the contractive response of rat jejunum to 15-methyl-PGF_2α. However, it increased the contractive response of uterus to 15-methyl-PGF_2α. Loperamide in dose of 2.5×10^-7 g/ml stimulated but in dose of 10^-6 g/ml inhibited the spontaneous contraction of rat uterus. Loperamide antagonized the action of 15-methyl-PGF_2α on increasing jejunum secretion. This antagonizing effect could not be reversed by naloxane. The LD₅₀ of Loperamide in mice was determined to be 77.9 mg/kg by intraperitoneal injection.