Recent evidence suggests that the hypothalamic ventromedial nucleus contains a wide variety of functionally specialized neurons, some of which are located within discrete regions of the nucleus. However, regional morphologic variations in neuronal populations within the HVM have not been demonstrated. In the present report, the neurons of the HVM were studied in young adult, male rats with the electron microscope. The collection and analysis of cytologic data were systematized with reference to location on a histologic map of the HVM, which was prepared prior to fine structural study.Neurons of the HVM were observed to exhibit regional variations in cytology. For example, cisternae of the rough endoplasmic reticulum in the anterior and dorsomedial regions of the nucleus are commonly collected into regular stacks typical of Nissl bodies. In contrast, rough cisternae in the posterior ventrolateral region of the HVM are often extremely long and pleomorphic, and form broad, loosely organized Nissl zones, in which occur myelin figures and wide varieties of cytoplasmic filamentous bodies.Other neurons located principally in the ventrolateral and posterior regions of the nucleus are characterized by the presence of whorled bodies. Still others restricted to the lateral rim of the nucleus are conspicuous by virtue of their enormous size and massive accumulation of Nissl material.One extremely unusual variety of HVM neuron is distinguished by the presence of large, acristic mitochondria, polysomal disaggregation, and aberrant nucleolar morphology. These cytologic features may indicate metabolic quiescence and perhaps anaerobiosis.Possibly these region-centered variations in neuronal cytology reflect the functional parcellation of the HVM. HVM participates in these diverse activities, however, is largely unknown.Some current theories propose that at least some HVM functions are mediated by functionally specialized cell types located in discrete areas of the nucleus. For example, there is evidence indicating that the HVM contains hypophysiotropic hormones, such as LHRH (Palkovits et al., '74 To date, the existence of specific populations of morphologically specialized neurons within the HVM has not been demonstrated. Previous anatomical studies of the HVM (Krieg, '32; Kaelber and Leeson, '67; Kovac and Denk, '68; Szentagothai e t al., '68; Millhouse, '73a; Takeichi and Noda, '74) have generally indicated a high degree of homogeneity among HVM neurons, which is somewhat surprising in view of the proposed diversity of functionally distinct cell types that are hypothesized to reside in this nucleus. The fact that Golgi-impregnated material did not reveal significant variation among HVM neurons may be attributed to the possibility that the morphologic features revealed by this method may not reflect the functional specializations of the HVM cell populations. Inasmuch a s many of the hypothesized functions of HVM neurons, such as glucoreception or neurosecretion, involve metabolic specializations, the morphologic paramet...