The islands of Calleja (IC) in the rate are composed of seven small groups of granule cells in the polymorph layer of the olfactory tubercle and one large group, the insula magna, which lies along the border between septum, nucleus accumbens and nucleus of the diagonal band. The cytoarchitecture and neuronal morphology of the IC and surrounding cells, studied using Nissl-stained and Golgi-Kopsch material, are described. In addition, the afferent and efferent connections of the IC were analyzed using fluorescence histochemistry, the autoradiographic tracing method, and the anterograde and retrograde horseradish peroxidase methods. Topographically organized projections to the IC from the dopamine-containing cells of the substantia nigra-ventral tegmental area are demonstrated by the glyoxylic acid fluorescence histochemical method and the autoradiographic tracing technique. Anterograde and retrograde horseradish peroxidase studies provide evidence for reciprocal, topographically organized interconnections between the IC and the septum, nucleus accumbens, amygdala and piriform cortex. These observations indicate that the IC constitute a unique population of granule cells, located in the olfactory tubercle, innervated by dopamine neurons of the mesencephalon and interconnected with olfactory and non-olfactory components of the basal forebrain.