The presence and distribution of cholecystokinin (CCK)/gastrin-like immunoreactive (IR) material were examined in the goldfish brain and gut. In the forebrain, CCK/gastrin-like IR fibers and perikarya were localized to nuclei of predominantly the ventral telencephalon and diencephalon; more dorsal forebrain regions contained only few, thin-beaded, sparsely distributed IR fibers. CCK/gastrin-like IR was specifically detected in the preoptic hypothalamus, including the nucleus entopeduncularis, nucleus preopticus periventricularis, and nucleus preopticus. Of all brain regions examined, the highest concentration of CCK/gastrin-like IR staining was consistently observed in nuclei of the ventroposterior and inferior lobes of the hypothalamus. Within the gut, both nerve fibers and endocrine cells contained CCK/gastrin-like IR. The abundance of CCK/gastrin-like IR material within brain areas known to represent the feeding center of fish, as well as the presence of IR material within the gut, suggests that CCK may influence feeding behavior in fish. When injected either intraperitoneally or into the third brain ventricle into goldfish, sulfated CCK-8 (CCK-8s) suppressed food intake. The nonsulfated form of CCK-8 was not as effective as CCK-8s in suppressing feeding after intraperitoneal injection. No consistent changes were observed in circulating serum growth hormone or gonadotropin levels after either intraperitoneal or intracerebroventricular injection of CCK-8s. These studies demonstrate for the first time that CCK-8s is effective in acutely suppressing feeding behavior in fish when administered either peripherally or centrally into the third brain ventricle.