Abstract The magnocellular neurones of the supraoptic nucleus which synthesize and secrete vasopressin and oxytocin have been commonly regarded as simple “output” neurones in that they receive an input, generate an action potential and in turn release hormone from their terminals in the posterior pituitary. Three lines of evidence are presented which suggest that rat supraoptic nucleus neurones also have axon collaterals which terminate in the hypothalamus close to the nucleus. Small injections of horseradish peroxidase were made directly into the nucleus in hypothalamic slices, allowing visualization of the axons of supraoptic neurones. Collaterals of these axons could be observed in regions both dorsal and dorsolateral to the supraoptic nucleus. In a separate series of experiments, sections of perfusion-fixed hypothalamus were stained for vasopressin and oxytocin using specific antisera. Peptide-containing collaterals of both types were observed near the supraoptic nucleus, in a region similar to that seen after horseradish peroxidase injections. Finally, electrophysiological studies were carried out on hypothalamic slices containing the supraoptic nucleus. A small concentric bipolar stimulating electrode was placed directly into the nucleus and activity of lateral hypothalamic neurones within 0.1–1 mm of the nucleus was recorded. Of 68 neurones studied, 52 were exicted by supraoptic stimulation via a synaptic pathway that could be blocked by Ca 2+ -free solutions containing 18mM Mg 2+. These studies suggest that supraoptic neurones communicate via axon collaterals with other neurones in the lateral hypothalamus, in addition to their previously well characterised functional role in neurosecretion.