Invitro experiments with strips of pregnant rat uterus, normal estrogen-primed rat uterus and duodenum showed that cyproheptadine HCl (Periactin®), a serotonin antagonist, inhibits the intensity of spontaneous contractions of these muscle strips. The degree of inhibition depends upon the concentration of cyproheptadine. Contraction frequency was also reduced in these smooth muscles, the basic tonus not being affected. Addition of serotonin (5-HT) to the water bath increases the intensity of muscle contractions. Cyproheptadine inhibits this increase at smaller doses than those required for inhibiting spontaneous contractions. Whereas cyproheptadine does not inhibit completely the pace-maker rhythm of the muscle cells, it suppresses the amplitude of their contractions.