1. Potassium channel opening drugs (KCOs) include benzopyrans such as cromakalim, cyanoguanidines such as pinacidil and tetrahydrothiopyrans such as RP 49356. 2. While clinical trials have indicated that cromakalim may be of benefit in the treatment of nocturnal asthma, it remains to be determined whether KCOs will find a place in our armamentarium of clinically useful anti-asthma agents. 3. KCOs inhibit the spontaneous tone of airways smooth muscle in vitro, an action associated with membrane hyperpolarization towards the potassium equilibrium potential and with the promotion of 86Rb+ or 42K+ efflux from the muscle cells. KCOs suppress spasm of airways smooth muscle evoked by low (< 40 mM) but not high (> 40 mM) concentrations of KCl. Their relaxant effects in airways smooth muscle can be attenuated by a variety of agents (including sulphonylureas) known to inhibit the opening of plasmalemmal K(+)-channels. 4. The KCOs open an ATP-sensitive K(+)-channel (KATP) in the plasmalemma. KATP is not open under normal circumstances and does not play an important role in determining the strong outward rectifying behavior of the cell membrane. The biochemical mechanisms by which the KCOs promote the opening of KATP remain to be elucidated but probably do not involve channel phosphorylation consequent to the intracellular accumulation of cAMP. 5. By causing hyperpolarization of the plasmalemma, the KCOs inhibit the cellular influx of Ca2+ through voltage-dependent channels. Relaxation follows both as a direct consequence of the fall in cytosolic free Ca2+ and also as a consequence of reduced production of phosphoinositide second messengers. The KCOs may also inhibit Ca2+ uptake by, and hence Ca2+ release from, the sarcoplasmic reticulum. 6. KCOs can inhibit cholinergic and non-adrenergic, non-cholinergic (NANC) excitatory neuroeffector transmission in the airways by glibenclamide-sensitive mechanisms which may involve inhibition of neurotransmitter release. The KCOs do not attenuate NANC inhibitory neuroeffector transmission, suggesting that KATP may not be expressed in neurones of this type. 7. The active enantiomer of cromakalim has been found to be effective in alleviating nocturnal asthma at plasma concentrations just threshold for relaxing human airways smooth muscle in vitro. The clinical efficacy of cromakalim may therefore depend on an action other than the direct relaxation of airways smooth muscle. Animal studies indicate that KCOs can reduce airway hyper-reactivity at sub-bronchodilator doses. The mechanism of this effect remains to be elucidated and may not crucially depend upon inhibition of neurotransmitter release within the lung.