Supraoptic nuclei in the hypothalamus of rats were fixed for the electron microscope by vascular perfusion with solutions of glutaraldehyde followed by post fixation with osmium tetroxide. Cytochemical methods for detection of acid phosphatase and thiamine pyrophosphatase activity have been applied to glutaraldehyde-fixed frozen sections containing the neurosecretory cells. The enzyme activities have been localized to certain Golgi cisternae. Acid phosphatase activity is present in the large (0.4 ~ to 1.0 #) granules or dense bodieswhich are surrounded by a single limiting membrane; both features characterize these structures as lysosomes. Smaller (0.1 #) granules also present in the perikarya are generally unreactive towards enzyme activity and resemble in form the neurosecretory granules in the neurohypophysis.A prevalent finding common to recent electron microscope (EM) studies of neurosecretory cells has been the demonstration of secretion granules varying in size and internal structure within the neuroplasm. Palay (29) has reported the appearance of secretory material, in cells of the preoptic nuclei of goldfish, in the form of dense droplets of two sizes, roughly 0
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