Amiloride, an inhibitor of the Na+-H+ antiporter in airway epithelium, has been proposed as nebulized therapy to loosen secretions in patients with cystic fibrosis. Studies with bovine and canine trachea have determined that amiloride produces airway relaxation in vitro, suggesting that it may also have potential benefit as a bronchodilator. However, amiloride has not been shown to relax guinea pig trachea. Since guinea pig trachea is felt to be a useful model of human airway, we hypothesized that human airway response to amiloride would be similar to that of guinea pigs. Newborn guinea pig along with mouse tracheal responses to amiloride were compared to those of human fetal airway. Airway ring segments were constricted with an EC50-75 dose of acetylcholine and changes in isometric force to cumulative additions of amiloride (10-8 to 10-4 M) were recorded. In guinea pig airway with intact epithelium, amiloride produced contraction, increasing tension at maximum by 26±4%(n=16). Removal of epithelium did not alter this response (41±10%, n=8, p=0.12). Mouse trachea also responded with an increase in tension(10±6%, n=4). Likewise, human fetal airway tone increased by 14±9% [n=6 from 2 fetuses (16 wk gestation)]. In summary, unlike adult bovine and canine trachea, human fetal airway contracted upon exposure to amiloride. Guinea pig and mouse airways also contracted, and this contraction was independent of epithelium. These results, along with previous work employing furosemide, support the notion that guinea pigs and mice are useful paradigms in the study of ion transporters in the regulation of human airway tone. Funded by: US Army HSC, CRC, and NIH HL-45220.