The author measured roentgenologically the time needed for complete evacuation of 50% hypaque sodium, following instillation into the maxillary sinus through the lateral bony wall of the inferior meatus. Bilateral instillations were carried out in 15 normal subjects and unilateral instillations were peformed in 60 individuals with chronic suppurative maxillary sinusitis. The evacuation time required in relation to the normal and pathologic sinus mucosas were measured and then discussed the relations between the condition of the natural ostium and the histopathological types of the sinus mucosa. The following results were obtained ; 1) 50% hypaque sodium was completely evacuated from the maxillary sinus within 12 hours in 80% of the whole control group. 2) Of these 60 cases 22(33%) had good ventilation of the maxillary ostium, 34(55%) had poor ventilation and remaining 4(7%) were undetermined. In those 22(38%) individuals with chronic suppurative maxillary sinusitis, in which 50% hypaque sodium within 24 hours. Of the 34(55%) subjects with poor ventilation of the maxillary ostium, 28(45%) requires 36 hours or more for complete clearing. 3) All histopathological types of chronic suppurative maxillary sinusitis (edematous, infiltrative, fibrotic and mixed) showed more delayed drainage of 50% hypaque sodium from the maxillary sinus than those with normal sinus mucosa. In particular, the fibrotic and mixed types showed marked delayed in the drainage of 50% hypaque sodium from the maxillary sinus. 4) The number of cases in the this study are not sufficient to assert a conclusive relationship between the pathologic state of the sinus mucosas and the drainage time of instilled 50% hypaque sodium : however, the study would suggest a relationship between poor ventilation and delayed drainage. The drainage was least delayed in the edematous type and most delayed in the mixed type.