Eight Merino and eight Dormer ewes, none of which had been on heat for 2–3 months prior to treatment, were given six intra-muscular injections of 30 mg. progesterone at 3-day intervals. The ewes were divided into four groups, each consisting of two Merinos and two Dormers. On the second day after the last progesterone injection, 1000 i.u. P.M.S. were administered to each of the ewes in group I. The remaining ewes each received an injection of 500 i.u. P.M.S. on either the first and second days (group II), the second and third days (group III), or the first and third days after the end of the progesterone treatment (group IV).

Five of the Dormer ewes, but not one of the Merino ewes, were served after the treatment. Mean ovulation rates for the four groups were 2·50, 2·75, 2·00 and 2·50, respectively, and the mean numbers of large follicles (i.e. those which were observed at laparotomy, approximately a week after treatment, to have a diameter of not less than 4 mm.) were 3·00, 2·75, 1·00 and 2·00, respectively. Contrary to expectation, it was observed that only one of the ewes (a Merino) showed evidence of a silent ovulation shortly before treatment.

It is suggested that (i) the amount of oestrogen produced by the ovarian follicles of the Merinos may have been exceptionally low, thus accounting for the non-occurrence of oestrus in these ewes, (ii) ovulation rates might be increased by greater reductions in the time between the last progesterone injection and P.M.S. treatment, and (iii) if silent heat in the Merino is rare, it may be simpler to induce coincident heat and superovulation in ewes of this breed by two injections of P.M.S. at 16-day intervals.