The replication of influenza A0/PR8 virus started and continued simultaneously in all parts of the respiratory tract, without any preferential susceptibility of any area of the epithelial lining when the virus was administered to mice in a sublethal dose in the form of aerosol. After intranasal instillation, the initial virus replication in the lung tissue preceded by 4 to 8 hours the rise in infectious virus titre in the trachea and by 21-24 hours the rise in the virus titre in the nasal mucosa. Under conditions of aersol inhalation, the mice represent a suitable model for pathogenetic studies.