The succinic dehydrogenase (SDH) activity in the supraoptico-neurohypophyseal system of the rat was studied by means of electron microscopic histochemistry aiming at the explanation of earlier findings pointing out the consistently negative reaction of this enzyme in the axon terminals. SDH activity was found to be localized in the mitochondria of neurosecretoy neurons and pituicytes. The hypothalamic as well as the neurohypophyseal portions of neurosecretory axons have markedly stained mitochondria. In contrast to this, axon terminals devoid of neurosecretory material contain non-reacting mitochondria similarly to the previously described distribution of SDH activity in other regions of the nervous system. Accordingly SDH activity could be revealed neither in the mitochondria of axon terminals synapsing on the neurosecretory perikarya nor in the mitochondria of those neurohypophyseal boutons lacking neurosecretory granules. The more uniform distribution of SDH-activity in neurosecretory than in non-neurosecretory neurons points to the possible role of axoplasmic flow in causing within the neurons the described proximodistal decrease of SDH-activity.