The relationship between contamination with Bordetella bronchiseptica or Pasteurella multocida and the age of the piglets on the one hand and the pathogenesis of atrophic rhinitis on the other, is discussed. Minimal disease-free piglets born and reared on farms on which pigs affected with clinical atrophic rhinitis were present, developed clinical atrophic rhinitis at the age of six weeks. Piglets born in the Central Veterinary Institute (CVI) of sows from herds including pigs affected with atrophic rhinitis and reared under normal CVI conditions, did not develop clinical atrophic rhinitis. These sows are believed to be factors of minor importance in the pathogenesis of clinical atrophic rhinitis in piglets. When minimal disease-free piglets were contaminated during the third or fourth week of life, a slight degree of foreshortening of the upper jaw was observed in three out of nine pigs, the carcase weights of these animals not differing markedly from those of normal swine. When they were contaminated at the age of eight weeks, clinical atrophic rhinitis did not appear. It is essential that young piglets should be protected against atrophic rhinitis during the first few weeks of life.