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Cholecystokinin is a hormone that promotes gallbladder contraction and belongs to the group of 'silent hormones' (so called by JORPES) that control digestion in the upper part of the gastro-intestinal apparatus.IVY and OLDBERG first provided evidence that the cholecystokinetic effect of duodenal mucosa extracts is due to a specific hormone, of which they obtained the first samples. But IVY and OLDBERG'S cholecystokinin, as well as pancreozymin found many years later by some English workers (DUNCAN et coll.), turned out to be of little practical value; the preparations moreover caused undesirable side effects. In 1 9 5 4 , J o~~~s and MUTT finally elaborated a method of obtaining highly purified and active preparations, suitable for clinical purposes. The biologic assay of the hormone is made according to IVY and OLD-BERG or by closely related methods. The IVY dog-unit is defined as the amount of dried material, free of vasodilatatory effects, which, when dissolved in normal saline solution and injected intravenously within 10 or 15 sec, causes within the first three minutes a n intra-gaIlbladder pressure rise of 1 cm. The cholecystokinetic effect of cholecystokinin is demonstrated in vivo as well as in vitro.The release of the hormone from the duodenal mucosa seems to be connected with the stimulation of a procainsensitive receptor which is connected to the cells producing cholecystokinin by a pathway that can be blocked by hexamethonium (HONG, MAGEE and CREWDSON 1956).Cholecystokinin also causes the relaxation of the sphincter of Oddi; an effect