Pseudopregnancy (PSP) in the rat was significantly lengthened by a single injection of 20 μg LH or 100 μg FSH on Day 5. Doses of oestradiol ranging from 0·05 to 0·20 μg, injected at this time, also significantly prolonged PSP, whereas larger amounts did not. Neither the antioestrogen ICI 46474, nor progesterone, given early in PSP shortened its duration, the antioestrogen itself significantly extended PSP. Daily injections of 20 μg LH to pseudopregnant animals, starting on Day 5, maintained luteal function even longer than did the single dose. Hysterectomy of LH-treated animals did not further extend PSP indicating that the uterus was not involved in terminating luteal function in these animals. Oestrone, oestradiol and progesterone were measured on Day 8 of PSP, in the ovarian venous blood collected from animals treated with ICI 46474 or progesterone on Day 3, and from others subjected to hemiovariectomy and either X-irradiation or sham irradiation on that day. These measurements failed to reveal suppression of oestrogen secretion and consequent reduction in the secretion of other steroids due to the treatments. It is suggested that oestrogens may play an important rôle in the maintenance of luteal function during PSP by complementing the luteotrophic action of the gonadotrophins.