The molecular mechanisms of migraine pain have not yet been clarified. Neurogenic inflammation and a subsequent plasma extravasation in the dura mater have been suggested as causative factors. However, monoamine and peptide neurotransmitters involved in neurogenic inflammation do not cause significant head pain. Based on our previous studies of headache induced by i.v. infusions of glyceryl trinitrate (exogenous nitric oxide [NO] donor) and histamine (which liberates NO from the vascular endothelium), it is suggested that NO is a more likely candidate molecule. The present review examines the biology of this small messenger molecule, and the scientific evidence suggesting that it may play a key role in migraine headache. It is hypothesized that the release of NO from blood vessels, perivascular nerve endings or from brain tissue is a molecular mechanism which triggers spontaneous migraine pain. Furthermore, it has been shown that this hypothesis is supported by the recent findings that i.v. infusion of the NO synthase (NOS) inhibitor is effective in the acute treatment of migraine attacks. These novel observations indicate possible new approaches to the pharmacological treatment of migraine.