The Pontamine Blue reaction is associated with increased vascular permeability at the site of blastocyst attachment in the uterus. New Zealand Romney ewes of different ages were killed 15 minutes after an intravenous injection of that volume of a 0.5% solution of Pontamine Sky Blue 5BX in normal saline which was equivalent to 10% of the recipient's blood volume. A positive reaction on the 15th day of gestation confirmed the findings of Psychoyos (1967) [Adv. Reprod. Physiol. 2, 257] that microscopic evidence of implantation is apparent within 24 hours of the onset of the reaction, and the timing of embryonic attachment described previously [see V.B. 39, abst. 5212]. It is suggested that decidua formation may be linked temporally, but not causally, with the increased vascular permeability demonstrated by the Pontamine Blue reaction. GH.