Microscopic observations on diapedesis of the mesenterium in a rabbit was carried on by high speed cinephotography, where recording rate to 2200 f. p. s. were used. The majar features of diapedesis elucidated in this study were as follows: An erythrocyte which come near to a small opening on the microvascular wall transformed and was attracted by the opening. Then it extravasated from the vascular wall. The length of time required for the erythrocyte to pass through the wall ranged from 0.047 sec. to 0.31 sec.. The erythrocyte escaped with a high speed (5-7mm/sec.) as a dash at the external border of the vascular wall
(comparing 110μ/sec. of erythrocyte flow). Platelet flew toward the opening without showing down its flow velocity. Diapedesis meant in this study that erythrocytes extravasated one by one from the small openings proper to diapedesis. To author's knowledge, there is no study concerning with diapedesis observed with a high speed cinema. This study was read at the 8th Japanese Association of Angiology (1968) and the 15th Japanese Symposium of Medicine (1970).