Four trials, involving 245 ewes, were undertaken in attempts to reduce the prolonged hormone injection procedure necessary in the application of repeated progesterone-p.m.s. therapy. A continuous-injection apparatus was used as a possible means of rendering progesterone administration simpler. Information was sought on the oestrous and conception response where the first injection of p.m.s. was not preceded by progesterone therapy. Previous studies having indicated that oestrogen-progesterone balance may be important in the development of a suitably receptive uterine environment, two trials were conducted in which oestrogen therapy was combined with progesterone-p.m.s. treatment.

Trial 7. The use of a continuous-injection apparatus was studied in sixty-nine ewes, of several breeds and ages, in four farm flocks. Apparatus were used to administer progesterone in propylene glycol solution over periods of 7–12 days. They were adjusted to give an average daily output of progesterone ranging from 5 to 15 mg., administration being made subeutaneously. In twenty-five the function of these over the planned period of administration was considered satisfactory. A subcutaneous injection of 750 i.u. of p.m.s. was given when the apparatus was removed. Twenty-five per cent of the ewes mated after the treatment; of these, only 25% conceived and produced lambs. The poor response would appear to indicate that low dosage level parenteral progesterone therapy may be much less effective in facilitating oestrus and the conditions necessary for conception than the administration of single doses of greater magnitude at daily or longer intervals.

Trial 8. Fifty-one Clun ewes, of mixed ages and non-lactating, were arranged in two comparable groups. One group (twenty-five ewes) received an injection of 750 i.u. p.m.s. prior to the commencement of progesterone-p.m.s. therapy; the remaining twenty-six animals received progesterone-p.m.s. only. Combined treatment consisted of three doses of 50 mg. progesterone at 3-day intervals, with 750 i.u. p.m.s. given 2 days after the final progesterone injection. In animals receiving two injections of p.m.s., the interval separating such treatments was 15 days. Of the twenty-five ewes receiving prior p.m.s. treatment, fifteen (60%) mated, as compared with 54% of those which did not receive prior treatment. For ewes receiving two doses of p.m.s., the conception percentage was thirty-three, as compared with forty-one in the controls. The results indicate that repeated progesterone-p.m.s. therapy may not be conveniently simplified by a treatment in which the first p.m.s. injection is not preceded, by progesterone.

Trial 9. Eighty-five Half-Bred maiden yearling ewes were arranged in two groups. In one, a single dose of 50 μg. oestradiol was administered 24 hr. prior to commencement of progesterone-p.m.s. therapy; the second group (control) received progesterone and p.m.s. only. In each group, 10 mg progesterone was injected daily for 12 days, and 750 i.u. of p.m.s. administered 24 hr. after cessation of such treatment. Sixty-one per cent of oestrogentreated ewes and 78% of controls mated. There was no significant effect on conception; 33% of those which mated after receiving oestrogen conceived as compared with 41% in controls.

Trial 10. Forty mature, lactating Clun ewes were observed after a treatment consisting of seven daily injections of 25 mg. progesterone, with a single dose of 750 i.u. p.m.s. given 2 days later. Thirty-six ewes mated, and eighteen were injected with a single dose of 50 μmg. oestradiol 3 days after service. Fifty per cent of the oestrogen-treated ewes and 11·1% of the controls conceived. Results suggest that oestrogen treatment following mating may facilitate conception by its action on the uterus.