There have been few Golgi studies dealing with the cytoarchitecture of the supraoptic nucleus (SON). This is due in part to resistance of supraoptic neurons to impregnation by Golgi methods. In this study, the structure of the SON was examined in normal S/D rats by using both Nissl and Golgi-silver methods. The purpose was to correlate shape, size and location of neurons within the SON as revealed by these two techniques. On the basis of size, neurons of the SON can be divided into 3 populations: greater than 200 micron2, (9%); 100-200 micron2, (64%); and less than 100 micron2, (27%). The larger neurons are located predominantly at mid-nuclear levels; the smaller at rostral and caudal levels of the nucleus. The perikarya of most SON neurons (64%) are only slightly elliptical in cross-section (L/W less than or equal to 2). The large neurons, however, tend to be more spherical whereas the smaller neurons are more elongated (L/W greater than or equal to 3). In Golgi preparations, a variety of randomly distributed bipolar and multipolar neurons were identified. One form of bipolar neuron had a large spherical or oval cell body that was intimately associated with blood vessels. Its thick, varicose dendrites usually lacked spines and were not extensively branched. A second form of bipolar neuron was distinguished by its smaller more fusiform cell body and lengthy dendrites which were often spinous and more extensively branched. Axons, when present emerged from the cell body or a proximal dendrite and were uniformly thin except for fusiform swellings along their length. Among multipolar neurons, the following variants were distinguished: spherical and polygonal neurons of various sizes with 3-5 dendrites and small triangular neurons with dendrites arising from each of the poles. The results of this study demonstrate the heterogeneity of the rat SON and of its neuronal components, some of which appear suited to function in a nonendocrine capacity, possibly as interneurons.