Antifertility effects of prostaglandin F2α have been described in rats, hamsters, rabbits and monkeys (see Duncan & Pharriss, 1970, for references). However, it is not known whether or not they are due to inhibition of implantation. In rats, hamsters and perhaps other species, a localized increase in vascular permeability to macromolecules occurs at the site of blastocyst—uterine interaction (Psychoyos, 1966). In view of the fact that oedematous changes are known to occur at the site of implantation (Finn & McLaren, 1967) the vascular changes may include increase in vasodilation. The increased permeability and associated vascular changes can be demonstrated by intravenous injection of a macromolecular dye (e.g. Pontamine blue) and constitute one of the earliest signs of implantation (Finn & McLaren, 1967). Since part of the blood supply to the ovaries and uterus is derived from a common source and since F2α has been reported to