Carassin is a 21-amino-acid tachykinin-related peptide originally isolated from the goldfish brain. Carassin-immunoreactive (ir) perikarya were restricted to the nucleus preopticus periventricularis (NPP); immunoreactive perikarya were distributed sparsely in the rostral and caudal NPP, and were comparatively more in the lateral areas of the mid-NPP. Most perikarya showed thick axonal processes that extended into the rostral area of the Organum vasculosum lamina terminalis (OVLT) and terminated extensively in the vicinity of cells and blood capillaries in its ventral area below the preoptic recess; the OVLT may play a role in release of the carassin-ir material into the general circulation. The axonal terminals diminished caudally and were almost absent in the posterior region of the OVLT. Most epithelial cells of the olfactory organ were carassin-ir positive. Several fibers showing carassin-ir were also present in the olfactory bulb and are presumed to originate from the olfactory epithelium. Carassin-like ir granules were found in some cells of the proximal pars distalis (PPD) of the pituitary gland. Adjacent sections reacted with growth hormone (GH) and gonadotropin (GtH) antibodies, revealed that carassin coexists with GtH in a small percentage of cells. Further, there is sexual dimorphism in the nature and distribution pattern of carassin-ir granules in the PPD. In males, GtH cells contained a few carassin-ir granules; whereas, in females, GtH cells frequently had clusters of carassin-ir granules. These carassin-containing granules may participate in auto- and/or paracrine regulation of the pituitary. The ir-perikarya of the NPP may influence hypophysial function through a multisynaptic pathway. The functions of carassin in goldfish remain to be investigated.