A study has been made of the nasal histology in normal ferrets and in ferrets during and after infection with epidemic influenza virus. During the acute stage of infection the respiratory epithelium of the nasal mucous membrane undergoes necrosis with desquamation of the superficial cells and exudation into the air passages, and an inflammatory reaction occurs in the submucosa. Repair begins on the 4th day after infection, and from the 6th to the 14th day the respiratory area is covered successively by a transitional, a stratified squamous, and finally a stratified columnar epithelium. By the 21st day after infection the epithelium has been largely restored to normal but repair in the submucosa and cartilage is still in progress. The respiratory mucosa is substantially normal in structure 1 month after infection although minor abnormalities of cellular arrangement and type can still be distinguished.