Substance R is a biologically active substance extracted by us from the bovine brain. This substance could also be extracted from the rabbit brain. It is assumed, therefore, that Substance R may exist not only in the bovine brain but also in the brains of other animals. Substance R extracted from the rabbit brain was similarly effective to that from the bovine brain in eliciting relaxation of the isolated guinea-pig taenia coli. Moreover, this substance caused relaxation of the isolated human taenia coli.