An ultrastructural comparison between the nasal cavities of healthy rabbits and those suffering from two forms of spontaneous infection with Pasteurella multocida was undertaken. Twelve commercially produced rabbits of different ages and respiratory health status were divided into four groups: healthy from 0 to 21 days (G1, n=2); healthy from 23 to 49 days (G2, n=2); healthy from 51 to 69 days (G3, n=2); diseased rabbits with septicemia and the rhinitic form of P. multocida infection (G4, n=3). The main ultrastructural changes observed were a widening of the interepithelial spaces, increased activity and number of goblet cells, the formation of two types of vacuoles in epithelial cells, the degranulation and migration of heterophils between the epithelial cells, and the association of this migration with some of the other changes. No bacteria were observed adhering to the epithelium, and very few were observed free in the mucus. Scant inter-epithelial spaces were found in healthy rabbits, but they were not as large and numerous as those found in diseased animals. We discuss the origin and meaning of these changes but, we focus on the significance of the inter-epithelial spaces and goblet cells for the defense of the upper respiratory airways against the bacterium and its lipopolysaccharide.