The distribution of gastrin-cholecystokinin-like peptide(s) is reported in brain and hypophysis of the rat. The unlabeled peroxidase-antiperoxidase complex immunohistochemical technique was used. Controls of specificity for various peptides were studied with solid-phase absorption. Colchicine treatment was necessary to obtain positivity in many neuronal cell bodies. In addition to their already known distribution, gastrin-cholecystokinins containing neural cell bodies and fibers were present in olfactory structures, in various preoptic and hypothalamic nuclei (except in mamillary bodies), in mesencephalic nucleus linearis rostralis, and in A-10, A-9, and A-8 regions of Dahlström and Fuxe, which include substantia nigra. From previous investigations and the present distribution study, it can be inferred that, although most of the brain material consists of cholecystokinin, gastrins may also be present in hypothalamo-posthypophyseal magnocellular cells, in nucleus tractus solitarii, and in the dorsal horn of the spinal cord. The distribution of positive cell bodies in the peripheral part of the paraventricular nucleus and in the dorsal part of the supraoptic nuclei in the hypothalamus is similar to that of oxytocin neurons. The localization of positive cell bodies in A-10, A-9, and A-8 regions of Dahlström and Fuxe is similar to that of dopaminergic neurons. The mesencephalic concentration of cell bodies and the wide distribution of fibers in striatal, hypothalamic, septal, and other hemispheric structures together with thick positive fibers in the medial forebrain bundle is consistent with the existence of ascending mesencephalic pathways, including the nigrostriate pathway.