Magnocellular neurosecretory cells are cells within the supraoptic nucleus and paraventricular nucleus. They are also found in smaller numbers in accessory cell groups between these two nuclei, the largest one being the nucleus circularis. The magnocellular neurosecretory cells have long axons which comprise the posterior pituitary. There are two types of magnocellular neurosecretory cells, oxytocin-producing cells and vasopressin-producing cells, but a small number can produce both hormones.