The recent discovery of nitric oxide (NO) as an intercellular messenger has brought about surprizing impacts in many branches of medicine and life science. This article reviews such developments in neuroscience where the simple gas molecule acts as an endogenous neuro-transmitter. The basic scheme of NO-mediated neural signalling is explained, along with examples illustrating new understanding of synaptic plasticity such as long-term potentiation. Also reviewed is the enzymological principle which commonly underlies the NO functions in physiology, immunology and oncology. Possible influence of NO on neural network modelling is discussed briefly.