1 Opioid agonists inhibit neurogenic mucus secretion in the airways. The mechanism of the inhibition is unknown but may be via opening of potassium (K + ) channels. We studied the e ect on neurogenic secretion in ferret trachea in vitro of the OP 1 receptor (formerly known as d opioid receptor) agonist [DPen 2,5 ]enkephalin (DPDPE), the OP 2 receptor (formely k) agonist U-50,488H, the OP 3 receptor (formerly m) agonist [D-Ala 2 , N-Me-Phe, Gly-ol 5 ]enkephalin (DAMGO), the ATP-sensitive K + (K ATP ) channel inhibitor glibenclamide, the large conductance calcium activated K + (BK Ca ) channel blocker iberiotoxin, the small conductance K Ca (SK Ca ) channel blocker apamin, the K ATP channel opener levcromakalim, a putative K ATP channel opener RS 91309, and the BK Ca channel opener NS 1619. Secretion was quanti®ed by use of 35 SO 4 as a mucus marker. 2 Electrical stimulation increased tracheal secretion by up to 40 fold above sham-stimulated levels. DAMGO or DPDPE (10 mM each) signi®cantly inhibited neurogenic secretion by 85% and 77%, respectively, e ects which were reversed by naloxone. U-50,488H had no signi®cant inhibitory e ect on neurogenic secretion, and none of the opioids had any e ect on ACh-induced or [Sar 9 ]substance Pinduced secretion. 3 Inhibition of neurogenic secretion by DAMGO or DPDPE was reversed by iberiotoxin (3 mM) but not by either glibenclamide or apamin (0.1 mM each). Iberiotoxin alone did not a ect the neurogenic secretory response. 4 Levcromakalim, RS 91309 or NS 1619 (3 nM ± 3 mM) inhibited neurogenic secretion with maximal inhibitions at 3 mM of 68%, 72% and 96%, respectively. Neither levcromakalim nor RS 91309 at any concentration tested signi®cantly inhibited acetylcholine (ACh)-induced secretion, whereas inhibition (60%) was achieved at the highest concentration of NS 1619, a response which was blocked by iberiotoxin. 5 Inhibition of neurogenic secretion by levcromakalim (3 mM) or RS 91309 (30 nM) was inhibited by glibenclamide but not by iberiotoxin. In contrast, inhibition by NS 1619 (30 nM and 3 mM) was blocked by iberiotoxin but not by glibenclamide. 6 We conclude that, in ferret trachea in vitro, OP 1 or OP 3 opioid receptors inhibit neurogenic mucus secretion at a prejunctional site and that the mechanism of the inhibition is via opening of BK Ca channels. Direct opening of BK Ca channels or K ATP channels also inhibits neurogenic mucus secretion. In addition, opening of BK Ca channels inhibits ACh-evoked secretion of mucus. Drugs which open BK Ca channels may have therapeutic anti-secretory activity in bronchial diseases in which neurogenic mechanisms and mucus hypersecretion are implicated in pathophysiology, for example asthma and chronic bronchitis.