1 6-hydroxydopamine (6-OHDA) reduced the noradrenaline content by 50% in the proximal branches of the canine mesenteric artery without significant change of dopamine content. For the main trunk from the same blood vessel a parallel depletion of dopamine and noradrenaline was found to occur. 2 In the proximal branches of control animals 43% of nerve profiles have a very high proportion of small dense cored vesicles (SDCV), whereas in the remaining nerve profiles the proportion of SDCV and large dense cored vesicles (LDCV) was about the same. In the main trunk 89% of nerve profiles have a similar proportion of both types of vesicles. 3 In the proximal branches, 6-OHDA drastically reduced the number of nerve profiles with a high proportion of SDCV, thus resulting in a preferential disappearance of SDCV. In the main trunk, 6-OHDA produced a parallel reduction of both SDCV and LDCV. 4 The results presented suggest that the 6-OHDA-insensitive dopamine pool found in the proximal branches of the mesenteric artery is mainly localized in LDCV, whereas SDCV appear to be the major store of noradrenaline.