The isolation of a smooth muscle stimulating substance from cow's placental tissue was studied. This active substance behaves like an unsaturated fatty acid. In paper chromatogram the spot of the final active fraction locates on that of linoleic acid. But it is more higher unsaturated acid than linoleic acid. The addition of 0.016μeg. of the final acidfraction stimulates the rabbit's jejunum suspended in 10ml. of Tyrode solution. The muscle stimulating action of this substance is antagonized by papaverine and atropine but not by dibenamine. This placental substance differs from any muscle stimulating lipid-soluble acids which have been so far reported.
