The Pontamine Blue reaction (Finn & McLaren, 1967) reflects the increased vascular permeability at the site of blastocyst attachment (Psychoyos, 1960) which presages cellular changes in the uterus associated with implantation of the embryo (Finn & McLaren, 1967; Boshier, 1968, 1969). Leakage of this macromolecular dye, therefore, precedes any modifications in uterine histology that can be associated with attachment of the embryo. The increase in vascular permeability has been considered a sine qua non for decidua formation (Psychoyos, 1961). In this study, the Pontamine Blue reaction was used (1) as confirmation of earlier observations (Boshier, 1968) that implantation in sheep occurs on the 16th day of pregnancy, and (2) to determine whether this reaction is present in sheep, a species in which there is no evidence of decidual cell formation preceding the uterine epithelial changes which occur at the implantation site (Boshier, 1968, 1969). New Zealand Romney ewes of different ages were killed 15 min after an intravenous injection, lasting over 60 sec, 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 [calculated as (lb body wt×0·07 litres)/2·2]. Uteri containing extra-embryonic membranes were examined under cold normal saline, and the caruncular and inter-caruncular areas were compared for difference in colouration. Only those uteri in which there was a differentially