Frusemide, an inhibitor of sodium-potassium-chloride (Na-K-Cl) cotransport, has been shown to inhibit the airway response to several constrictor stimuli in asthmatic subjects. The protection seen with frusemide in these studies could be due to an effect on epithelium, inflammatory cells, neural pathways, or airway smooth muscle. To determine whether frusemide inhibits airway smooth muscle contraction, experiments were performed in bovine and human airways in vitro. Fresh bovine tissue was obtained from the abattoir and human tissue from thoracotomy. The effect of 10(-5)M frusemide on histamine, potassium chloride, and hyperosmolar saline induced contractions was studied in bovine tracheal strips without epithelium. Frusemide, at a concentration that specifically inhibits Na-K-Cl cotransport, did not inhibit contraction caused by any of these agents. Frusemide was also without effect on hypertonic saline induced contractions of bovine and human bronchial rings with epithelium intact. These results suggest that modification of Na-K-Cl cotransport does not alter airway smooth muscle contractility and that the protective effect of frusemide on induced bronchoconstriction in vivo is unlikely to be due to a direct effect on airway smooth muscle.