After an intraventricular administration of colchicine, the arcuate nucleus of rat hypothalamus was examined light and electron microscopically by pre-embedding immunohistochemistry for somatostatin. The arcuate nucleus exhibited numerous immunoreactive cell bodies and dense networks of immunoreactive fibers. The fibers appeared to surround immunonegative cell bodies. The immunoreactive cell bodies were multipolar in shape and projected immunoreactive processes to some extent. The immunoreactive cell bodies and fibers received synaptic contacts by immunonegative fiber terminals containing a large number of synaptic clear vesicles. Similarly, immunoreactive somatostatin fibers appeared to terminate upon other immunonegative cell bodies and fibers. The immunoreactive presynaptic terminals contain several labeled granules and numerous synaptic vesicles. In close proximity to these immunolabeled terminals, non-labeled presynaptic terminals were also observed upon the immunonegative cell bodies and fibers. This suggests that in the arcuate nucleus neurons regulated by somatostatin neurons are also under the control of other types of neurons.