Neurosecretory nerve endings in the rat neurohypophysis release their hormones by exocytosis1 subsequent to an influx of calcium from the external medium2. The nerve endings are characterized by the presence of neurosecretory granules, mitochondria, occasional vacuoles, and a population of microvesicles similar in appearance to spherical synaptic vesicles. The function of the microvesicles has, for a long time, been uncertain. In view of evidence that coated microvesicles isolated from cerebral cortex are capable of ATP-dependent calcium accumulation3, a method has now been developed for the visualization of calcium in the neurohypophysis at the ultrastructural level. With this technique, calcium precipitates are consistently seen in the microvesicles, mitochondria and glial cell (pituicyte) nuclei. In addition, the pituicyte cytoplasm and perivascular space show a diffuse precipitate which can be removed by washing the tissue prior to fixation. The function of the microvesicles might therefore be to sequester calcium within the nerve endings.