Introduction A newly discovered class of messenger molecules appears to be concerned in a whole variety of biological functions in vertebrates: gaseous in nature, and thought of until recently as little more than toxic pollutants, these molecules have in fact become of considerable scientific importance. The notion of gases acting as biological signals was initially surprising, when it was proposed that nitric oxide (NO), or a closely related thiolic derivative, could account for the vasodilatory activity of endothelium-derived relaxing factor (EDRF) (Palmer et al. 1987). A rapidly growing body of evidence has since consistently indicated that NO or a related substance plays a role in such diverse functions as long-term potentiation (LTP), a neurophysiological model for the processes of learning and memory, as well as immune responses, and the autonomie activity underlying gut relaxation and penile erection subserved by non-adrenergic non-cholinergic (NANC) peripheral fibres (Moneada et al. 1991). Furthermore, there