The granule cell clusters in the rat olfactory tubercle were studied in Nissl-stained and Golgi-impregnated sections. Discrete cell clusters that vary in size and shape occur mainly in the multiform layer and less often in the molecular layer. In cell-stained sections they consist of small, round granule cells, 5-8 microns in diameter, that often surround a core or hilar area, which may contain larger neurons. In Golgi sections, the uni- or bipolar granule cells have a globular-shaped soma and varicose dendrites that are thin, have few branches, and are usually less than 100 microns long. The dendrites remain within the border of the cluster. There are few spines on most granule cells; however, a small population of granule cells is spine-rich. The axons are beaded, seldom have collaterals, and do not appear to exit from the cluster. Either in the hilus or in among granule cells are the special large hilar neurons, whose somata measure 15-17 x 18-22 microns. Unlike most of the neurons that are near a granule cell cluster, the dendrites, and perhaps axons, of the special large hilar neurons spread throughout a cluster. Differences in their dendrites suggest that there may be several varieties of them, but not enough examples have been studied to produce a useful classification. Some of their dendrites have bushlike terminal endings. Only the initial, beaded segment of their axons has been impregnated. Three types of afferent fibers have been identified: (1) Axons that are probably afferent to the olfactory tubercle course along a granule cell cluster giving off short collaterals that end in the periphery of a cluster. (2) Axon bundles that arise mainly from medium-sized densely spined neurons in the tubercle travel through a cluster, emitting boutons en passant or short collaterals that may end on granule cells. (3) Thick axons, which are among the thickest fibers in the olfactory tubercle, enter a cluster and develop a number of collaterals that in turn divide, and finally produce a unique terminal arborization in the cluster. The granule cell clusters are frequently identified as the islands of Calleja. A comparison of the structure of granule cells with that of the cells Calleja (La Region Olfactoria del Cerebro, Madrid: N. Moya, 1893) described in the "isolates olfativos," or islands of Calleja, indicates that he was pointing to the thickened, ruffled portions of the dense cell layer and not to the granule cell clusters.(ABSTRACT TRUNCATED AT 400 WORDS)