The distribution of the calcium binding protein calretinin (protein 10) was examined in the rat forebrain by immunohistochemistry. The main and accessory olfactory bulbs had immunoreactive label in granule, periglomerular, and mitral cells. Positive fibers were noted in the external plexiform and granule cell layers, glomeruli, and in the molecular layer of the anterior olfactory nucleus. The cerebral cortex contained calretinin label in nonpyramidal bipolar cells. Cells in the substantia nigra compacta and ventral tegmental area were also calretinin positive as were nigrostriatal and mesolimbic projections (caudate-putamen, nucleus accumbens). In the hippocampus, interneurons were stained in all the subfields of the CA1-CA4 regions. In the thalamus, many positive cells were observed in the periventricular, reticular, lateral habenula, and reunions nuclei. Calretinin immunoreactive cells were particularly abundant in the lateral mamillary and septofimbrial nuclei. Several fiber tracts were also revealed, i.e., the lateral olfactory tract, mamillothalamic tract, fasciculus retroflexus, optic tract, and stria medullaris. These results demonstrate a distinct distribution of calretinin within cell bodies and fibers.