In the caudate-putamen of the rat a patch/matrix organization can be recognized on the basis of the immunohistochemical distribution of several markers, which include enkephalin, substance P, dopamine, and calcium-binding protein. In the present experiments the distributional relations of these markers were investigated in the nucleus accumbens. The distribution of enkephalin fibers shows different inhomogeneities according to their location in the nucleus. Rostrally, heavily labeled areas stand out against a moderately stained background, whereas caudally, in medial and ventral parts of the nucleus, lightly stained areas delineate regions in the moderately stained neuropil. In the distribution of substance P, areas with high staining intensity were observed in the medial and ventral parts of the nucleus accumbens. Inhomogeneities in the distribution of strong dopamine immunoreactivity consist of weakly immunoreactive areas throughout the rostrocaudal extent of the nucleus accumbens and extremely heavily labeled areas in the medial and ventral parts of the nucleus. Calcium-binding protein immunoreactivity can only be detected in dorsal parts of the nucleus. The generally intense immunostaining for calcium-binding protein is interspersed with "blanks" of weak immunoreactivity. The heavily and moderately labeled enkephalin areas each maintain specific relations with inhomogeneities in the distribution of substance P, dopamine, and calcium-binding protein. Rostrally, the heavily labeled enkephalin areas coincide with areas strongly immunostained for calcium-binding protein and with lightly stained areas in the dopamine and substance P immunoreactivity patterns. In the same region lightly stained areas in the enkephalin distribution match heavily labeled substance P areas. Caudally, in the border region of the nucleus accumbens and the caudate-putamen, the heavily labeled enkephalin areas are either related to "blanks" or to the intense staining regions in the calcium-binding protein immunoreactivity distribution. The moderately labeled enkephalin areas caudomedially in the nucleus accumbens are in register with the heavily labeled regions in the distribution of substance P and with the extremely heavily labeled regions in the distribution of dopamine. Relations with connectivity are discussed and the inhomogeneities are compared to those in the caudate-putamen. It is concluded that in the ventral striatum either one patch and one matrix compartment exist with different immunohistochemical relationships or there are several compartments with different immunohistochemical characteristics and different input-output relations.