The distribution of the opioid peptide, dynorphin, has been studied in discrete, microdissected hypothalamic nuclei, and compared with the distribution of vasopressin. Both peptides were found in relatively high concentration in the supraoptic and paraventricular nuclei. However, dynorphin-like immunoreactivity (DYN-LI) was much more widely distributed in the hypothalamus than vasopressin-like immunoreactivity, with highest concentrations in the anterior hypothalamic and ventromedial nuclei. DYN-LI was also observed in some extra-hypothalamic structures; concentrations which were comparable to the highest levels in the hypothalamus were found in the tractus diagonalis and the nucleus interstitialis stria terminalis. Among the three brainstem nuclei examined, DYN-LI levels were highest in the sensory nucleus of the trigeminal nerve.