Summary. Rat blastocysts, flushed from the uterus at the moment of the initiation of the implantation process and observed in vitro by time\x=req-\ lapse cinematography, were seen to undergo a series of contractions and dilatations. By these movements, vesicles, containing granular material, emerged through the zona pellucida into the culture medium. In all species so far studied, the earliest known step in the series of interactions between the ovum and the endometrium which lead to implantation, is an increase in the permeability of the endometrial capillaries surrounding the blastocysts. In the rat, this early response of the sensitive endometrium to the presence of the blastocyst is manifested on the afternoon of the 5th day of pregnancy. As the volume of the blastocoele is considerably reduced in most of the blastocysts recovered from the uterus at this moment, it has been suggested that components of the fluid released from the blastocoele may constitute the stimulus for inducing the vascular changes in the endometrium (Psychoyos, 1967). In this paper, we report observations by time-lapse cinematography on the behaviour of blastocysts obtained from Wistar rats at noon on the 5th day of pregnancy (106 hr after the estimated time of ovulation) by flushing the uterine lumen with Tyrode. The culture medium was Eagle L medium containing 5% of foetal calf serum (DIFCO). After being rinsed three times in this medium, blastocysts were picked up in groups of four to five in a small drop of medium and were placed under paraffin oil in a plastic tissue culture dish; the culture dish was then placed under a microscope kept in an ambient temperature of 37° C and in a humid atmosphere containing 5 % C02 in air (Bitton-Casimiri & Psychoyos, 1968). A film at the rate of one frame per minute was taken, using a 16 mm Paillard Bolex camera, adapted to a time-lapse photographic apparatus (Wild). When recovered from the uterus, the blastocysts were still surrounded by an apparently intact zona pellucida. From the beginning of their life in vitro, they were seen to undergo a series of dilatations and contractions. These movements lasted for a period of about 48 hr, during which the zona pellucida ruptured and the blastocysts abandoning it started to dilate. During phases of dilatation, the volume of the blastocoele increased progressively over 5 to 6 hr, until the whole blastocyst reached a maximum size of about 150 µ diameter. A sudden, strong contraction, completed in less than 60 sec, then occurred, reducing the more...