King (1938) reported that the injection of human chorionic gonadotrophin (hcg) into rats on or before the day of implantation did not interfere with gestation, but that when given after implantation it resulted in the prolongation of pregnancy. Subsequently, Rosenfeld, Lapan & Baron (1941) confirmed that hcg inhibits parturition, but King's findings concerning the pre-implantation period remain unchallenged. The present work was designed to re-examine the effect of hcg on the pre-implantation stage of pregnancy in the light of the hypothesis that implantation in the rat must be preceded by an `oestrogen surge' stimulated by gonadotrophins (Shelesnyak, 1960; Psychoyos, 1963; Zeilmaker, 1963).
Nulliparous female Sprague-Dawley rats weighing 180 to 230 g were placed overnight with either normal or vasectomized males. Pregnancy or