A nonhuman transgenic mammal is described whose genome comprises a promoter construct operably linked to a heterologous DNA encoding an epithelial sodium channel .beta. subunit, wherein said promoter construct directs expression of the epithelial sodium channel .beta. subunit in lung epithelial cells of said animal, and wherein said transgenic mammal has increased lung mucus retention as compared to the corresponding wild-type mammal. The animal is useful in screening compounds for activity in treating lung diseases such as cystic fibrosis and chronic obstructive pulmonary disease.