Understanding how excitability of sensory neurons is regulated is an important goal since this excitability underlies pain transmission and unfortunately almost everyone will suffer from inflammatory pain at some point in their life. Recent studies have identified expression of M-type K+ channels (encoded by KCNQ genes) in damage-sensing (nociceptive) sensory neurons, where they are thought to control excitability. Accordingly, receptor-induced inhibition of M-current in these neurons has been shown to contribute to peripheral sensitisation and inflammatory pain. We have previously identified that oxidative modification of a triple cysteine pocket in the channel S2-S3 linker augments M-current. Here we describe a new mechanism for inhibition of M-current in nociceptors by nitric oxide donors. Nitrosylation is the addition of a nitrosyl group post-translationally to protein thiol groups, such as cysteine residues and has been shown to regulate the function of a large number of proteins. We show that in rat sensory neurons from the trigeminal ganglion, the nitric oxide donor, S-Nitroso-N-acetyl-DL-penicillamine (SNAP, 1mM), inhibited M-current by 52% ± 13 (S.E.M., n=13, p≤0.01, t-test) in 13/15 neurons measured using perforated-patch voltage clamp. We also inhibited currents using the structurally distinct nitrosyl donor compound diethylamine NONOate (30µM). We further studied the mechanism of this effect in CHO cells; SNAP inhibited currents elicited by KCNQ4-expressing CHO cells by 31% ± 4 (S.E.M., n=3, p≤0.05 t-test). Mutation of the triple cysteine pocket to alanines (CCC156-158AAA) abrogated this effect (n=4) but mutation of the cysteine in the C-terminus of KCNQ4 (C519A) did not (n=3). Since both inhibition of M-current and nitric oxide can induce acute pain, we propose that inhibition of M-current by nitric oxide may contribute to increased neuronal excitability and thus to NO-induced pain.