Nitric oxide is a labile gas which has been implicated in neuronal signalling. The enzyme responsible for the production of this molecule is present in the paraventricular nucleus of the hypothalamus, yet a specific role for nitric oxide in neurotransmission within this nucleus remains unclear. Using whole-cell patch-clamp recordings from paraventricular nucleus neurons in a coronal hypothalamic slice, we have assessed the acute effects of nitric oxide on membrane potential and ionic conductance. Recordings were obtained from 78 neurons with a mean resting membrane potential of -57.8 +/- 0.6 mV and a mean input resistance of 972 +/- 146 M omega. Cells were electrophysiologically classified into Type I or Type II according to previously established criteria. Bath application of nitric oxide (delivered either as a gas dissolved in solution, or liberated from the donor compound, N-acetyl-S-nitroso-D-penicillamine) elicited reversible membrane depolarizations (3 mV) in 14 of the 19 Type II cells tested. These cells also exhibited a decrease in input resistance following nitric oxide application. Similar effects were observed in response to bath application of L-arginine, with 11 of 14 cells displaying depolarizations and accompanying decreases in input resistance. Inhibition of nitric oxide synthase abolished the responses to L-arginine (n=2). The nitric oxide effects persisted when voltage-activated Na+ channels were blocked by tetrodotoxin (n=6). The depolarizations observed in Type II cells were mimicked by bath application of a membrane permeable cyclic GMP analogue (8-bromo-cyclic GMP) (n=8). Furthermore, nitric oxide depolarizations were abolished by pre-treatment of the slice with the guanylate cyclase inhibitor, LY83583 (n=4). Type I cells did not depolarize in response to nitric oxide (n=11). It is concluded that nitric oxide specifically depolarizes parvocellular neurons within the paraventricular nucleus via a mechanism that requires activation of guanylate cyclase and subsequent production of cyclic GMP. These findings provide the first insight into the cellular mechanisms underlying the acute effects of nitric oxide on neurons in the paraventricular nucleus.