The concept of two adrenergic receptor mechanisms or α- and β-adrenergic receptor systems, originally postulated by Ahlguist (1) is presently accepted. Furchgott (2) has presented evidence suggesting that there may be different types of receptors in the β- class. This conclusion was based on differences in relative potencies of isoprenaline, epinephrine, norepinephrine and phenylephrine in variety of isolated organs. On the other hand, some partial agonists in a series of cholinergic drugs contract the isolated taenia caecum (or taenia coli) of the guinea pig and at the same time the contraction which they produce is inhibited by their non-secific inhibitory action, so that maximal responses to partial agonists are smaller than that to full agonists (3-5). Furthermore, ephedrine which was classified as an adrenergic partial agonist relaxes the taenia caecum and at the same time the relaxation which it produces is antagonized by its own excitatory action, so maximal relaxation by it is smaller than that by a full agonist, epinephrine (5). DCI was also reported as an adrenergic partial agonist (6-8). However, mode of action of DCI is not precisely examined. The purpose of this paper is to compare the activities of some β-adrenergic blockers in various organs and to determine if there are different types of the β-adrenergic receptors in the organs. Furthermore, mode of action of DCI which is known as a dualist on some organs was precisely tested.