This study describes the ultrastructural and morphometric changes in the hypothalamic paraventricular nucleus (PVN) of the streptozotocin-induced diabetic rat. During the acute phase (3 days after induction of diabetes), dendrites were swollen and appeared to have a watery cytoplasm. The affected dendrites were postsynaptic to seemingly normal axon terminals. During the intermediate phase (1-9 months after induction of diabetes), the neuronal somata contained numerous vacuoles of various sizes which were probably formed from dilated rough endoplasmic reticulum. Several unidentifiable vacuolated profiles were also seen in the neuropil. During the late phase (12 months after induction of diabetes) electron-dense dendrites and axon terminals were readily encountered in the neuropil. Most of the electron-dense dendrites were postsynaptic to normal as well as electron-dense axon terminals. Some of the electron-dense axon terminals formed axosomatic synapses with seemingly normal somata. Perivascular glial cells in the neuropil were in the process of phagocytosis. At 1-12 months of diabetes, the sizes of the somata and nuclei of PVN neurons were significantly increased in comparison with age-matched control rats.