Axons from the histaminergic neurons of the tuberomammillary nucleus project to both the anterior and tuberal portions of the supraoptic nucleus. Histamine is known to activate vasopressin neurons via a histamine receptor subtype 1 and to increase release of vasopressin, but effects on oxytocin neurons have been previously unexplored. Here we investigated the effects of tuberomammillary nucleus electrical stimulation as well as of histamine antagonists on supraoptic nucleus oxytocin and vasopressin neurons in slices of rat hypothalamus. Electrical stimulation evoked short constant latency (approximately 5 ms), fast (4-6 ms onset to peak) inhibitory postsynaptic potentials in oxytocin neurons and, as shown previously, fast excitatory postsynaptic potentials in vasopressin neurons. These synaptic responses followed paired-pulse stimulus frequencies up to 100 Hz and were, thus, probably reflecting monosynaptic connections. Inhibitory postsynaptic potentials were selectively blocked by histamine receptor subtype 2 antagonists (either cimetidine or famotidine) and by picrotoxin but not by histamine receptor subtype 1 antagonists or bicuculline. Similar synaptic responses to tuberomammillary nucleus stimulation were found in 16 of 16 neurons immunocytochemically identified as oxytocinergic and in seven putative oxytocin neurons. Perifusion of the slice with low chloride medium (4.8 mM) reversed stimulus-evoked inhibitory postsynaptic potentials. We conclude that histaminergic neurons monosynaptically contact both oxytocin and vasopressin cells of the supraoptic nucleus and inhibit the former via activation of chloride channels which can be blocked by the histamine receptor subtype 2 antagonists, famotidine and cimetidine.