Dogs were used for experiment. The influence of cuff and blood pressure and duration of anesthesia upon the occurrence of pseudomembrane were studied. Six kinds of bacteria, which were found ordinarily in upper respiratory canal, were scattered in the trachea after endotracheal anesthesia. They were comprared with those, to which no bacteria were administrated. For the causal genesis of pseudomembranous larygotracheitis the infection of bacteria, which are ordinarily found in the upper respiratory canal, is seemed to play a main role, especially in dogs it occurs mostly by the haemolytic streptococci and yellow staphylococci. It tends to occur more frequently by the high cuff pressure, but we can not regard that it is caused only by the cuff pressure. The hypotention and duration of intubation showed no definite relation to its occurrence. Trauma at intubation, too high cuff pressure, too long duration of intubation, bad lubricant and low blood pressure etc. must have a secondary power to help its occurrence caused by bacterial infectiones.