A mouse-adapted influenza A virus, A/equine/London/1416/73-MA (H7N7) caused viral pneumonia, ganglionitis and encephalitis after intranasal inoculation in mice. Virological and pathological data suggested that this virus spreads to the brain by both hematogenous and transneuronal routes, and produces encephalitic lesions similar to those seen in mice infected with H5 highly pathogenic avian influenza A viruses by intranasal infection. Some mice infected with this strain were affected by aspiration pneumonia, which may be caused by neurogenic dysfunction of the pharyngeal/laryngeal reflex due to brain stem encephalitis.