Although certain of the prostaglandins are known to be potent smooth-muscle contracting agents (Horton, 1969), very little is known of their action on the mammary gland. Prostaglandin F2α (PGF2α) had no effect on the mammary epithelium of the mouse or on blood flow through the perfused goat udder (J. L. Linzell, personal communication; cited by Pickles, 1967). PGF1 had no effect per se on the milk-ejection pressure when given by close i.a. or i.v. injection in the lactating rabbit (Türker & Kiran, 1969) but did decrease the milk-ejection activity of oxytocin in the rabbit (Türker & Kiran, 1969) and abolished the milk-ejection activity of oxytocin in the rat (Haldar, Maiweg & Grosvenor, 1970). We now report results obtained during investigations into the effect of prostaglandins in the lactating guinea-pig assay for oxytocin (Tindal & Yokoyama, 1962). Changes in intramammary pressure were recorded after the close