Quantitative histological analysis of serial sections of the adult male rat brain gives mean total estimates of the numbers of cells in the magnocellular and parvicellular divisions of the paraventricular hypothalamic nuclei (PVN) of 6600 and 11,500, respectively. The numbers and densities (number/mm3) of presynaptic bouton profiles have been measured and calculated on electron micrographs of the magnocellular paraventricular nucleus (mPVN). There are approximately 18 x 10(6) presynaptic boutons in the magnocellular subdivision: 76% of the synaptic boutons are axodendritic, 17% axosomatic and 7% are unidentified but include a few axo-axonic contacts. One third of the boutons contain dense-core vesicles although their postsynaptic contacts do not differ from other boutons. The estimated ratio of the number of boutons per neuron in the magnocellular paraventricular nucleus is 2820:1.