After anticholinesterase treatment, the postsynaptic muscle membrane is depolarized by about 5 mV due to nonquantal release of acetylcholine (ACh) from the motor nerve terminal. This can be demonstrated by the hyperpolarization produced by the addition of curare (H-effect). The magnitude of the H-effect was decreased significantly to 3 mV when the nitric oxide (NO) donors, sodium nitroprusside (SNP) and S-nitroso-N-acetylpenicillamine (SNAP) were applied to the muscle, or when NO production was elevated by adding L-arginine, but not D-arginine, as a substrate. The H-effect was increased to 8-9 mV by inhibition of NO synthase by L-nitroarginine methylester (L-NAME), or by guanylyl cyclase inhibition by methylene blue and 1H-[1,2,4]oxidiazolo[4,3-a]quinoxalin-1-one (ODQ). ODQ increased the H-effect to 7.3 +/- 0.2 mV and diminished the SNP-induced decrease of the H-effect when applied together with SNP. The effects of NO donors and L-arginine were eliminated by adding reduced haemoglobin, an extracellular NO scavenger. The present results, together with earlier evidence for the presence of NO synthase in muscle fibres, indicate that nonquantal release of ACh is modulated by NO production in the postsynaptic cell.