This study describes ultrastructural and morphometric changes in the arginine vasopressin (AVP)-like immunoreactive and oxytocin (OT)-like immunoreactive neurons in the hypothalamic paraventricular nuclei (PVN) and supraoptic nuclei (SON) of streptozotocin-induced diabetic rats at 1-12 months post-diabetes. At 1-6 months post diabetes, both AVP-immunoreactive and OT-immunoreactive neuronal somata were hypertrophied in the PVN and SON. These neuronal somata contained highly dilated rough endoplasmic reticulum in the cytoplasm. The reaction product for AVP as well as OT localization was dispersed throughout the cytoplasm and cell nucleus, but not within the nucleolus. Moreover, the reaction product appeared to be studded onto the ribosomes on dilated cisterns of the endoplasmic reticulum. At 9-12 months post-diabetes, both AVP-immunoreactive and OT-immunoreactive dendrites contained dilated endoplasmic reticulum, autophagic vacuoles, lipid bodies, microtubules, membranous bodies and occasionally swollen mitochondria. Labelled hypertrophied axonal profiles containing neurosecretory granules, autophagic vacuoles, membranous bodies and tubulovesicular elements were also observed in the neuropil. Morphometric study showed that both AVP-immunoreactive and OT-immunoreactive neuronal somata of the PVN and SON in the diabetic rats were markedly hypertrophied at all the time intervals examined. It is concluded that the morphometric changes observed represent hyperactivity of both AVP- and OT-immunoreactive neurons, while the concurrent ultrastructural changes observed at later stages may be indicative of degeneration.