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Subcutaneous injection of the 5-hydroxytryptamine (5-HT) antagonist cyproheptadine hydrochloride (Periactin) produced a significant decrease in uptake of [14C]5-HT (serotonin) in the myometrium and to a lesser degree in the ovaries, foetuses and heart of pregnant rats. This effect of cyproheptadine was considerably increased in rats pretreated with the monoamine oxidase inhibitor pargyline hydrochloride. These results suggest that the anti \x=req-\ abortive effect of cyproheptadine is based on specific inhibition of the contractile effect of 5-HT on the myometrium. The hypothesis is advanced that cyproheptadine competes with serotonin for its receptors and thus blocks the effect of serotonin.