SUMMARY: In this study we have investigated the mechanism of action of leveromakalim and isoprenaline in human isolated airways with respect to the (K^{+})channels they activate and the possibility that these smooth muscle relaxants activate (\mathrm{K}^{+})channels on the airway epithelium. Mechanical removal of the epithelial layer (mean percentage of epithelium present (20 \pm 3 %, n=20) tissues) did not affect the relaxation responses to leveromakalim or isoprenaline, either in terms of maximal relaxation or sensitivity. Whilst having no effect on isoprenaline-induced relaxation, studied from basal tone, the ATP-sensitive (K^{+})channel blocker BRL (31660(10,30) and (50 \mu \mathrm{M})) reduced relaxation responses induced (from basal tone) by leveromakalim from (74 \pm 6 %) (of the maximal response to isoprenaline) to (48 \pm 12 %(n=) 7), (9 \pm 9 %(n=4)) and (0(n=4)), respectively. Charybdotoxin, a blocker of high conductance (\mathrm{Ca}^{2+})-activated (\mathrm{K}^{+}) channels, at concentrations of 30 and (100 \mathrm{nM}), had no effect on either leveromakalim-or isoprenaline-induced relaxation responses and yet charybdotoxin was active at (K_{C A}) channels in outside-out patches of hippocampal granule cells. Moreover, tetraethylammonium ( (10 \mathrm{mM}) ) inhibited neither isoprenaline- nor leveromakalim-induced relaxation. This study has demonstrated that the relaxation responses elicited in human bronchus to isoprenaline and leveromakalim are likely to be the result of direct effects on the smooth muscle with no contribution from epithelial receptors or (K^{+}) channels. The actions of levcromakalim appear to be mediated only via activation of (K_{\text {ATP }}) channels. Further, we have made the important observation that, under the experimental conditions of our study, isoprenaline does not activate the (K_{\mathrm{ca}}) channel to produce relaxation in human bronchus.