In the course of two breeding seasons 261 mature Romney ewes have been employed in three slaughter experiments designed to investigate the manner in which the reproductive performance of such animals, after treatment with a single injection of P.M.S., is conditioned by: (a) the amount of P.M.S. administered; (b) the nutritive status of the treated animals; (c) the stage of the oestrous cycle at which the P.M.S. is administered.

In the first experiment, one untreated control group and three experimental groups of ewes, treated with different amounts of P.M.S., on the twelfth and thirteenth days of the cycle, were used to establish a dose level-ovulation response relationship. Mean ovulation rates and ranges of ovulations observed were: control 1.17 (1–2); 250 i.u. P.M.S. 1.50 (1–2); 500 i.u. P.M.S. 2.07 (1–3); 1000 i.u. P.M.S. 4.33 (1–13).

In the second experiment, oestrous cycle lengths, ovulation and conception rates, and loss of ova during the first few weeks of pregnancy were compared in four groups of ewes that had been treated as follows: (a) neither flushed nor treated with P.M.S.; (b) flushed but not treated with P.M.S.; (c) not flushed but treated with 650 i.u. P.M.S. on the twelfth or thirteenth days of the cycle; (d) flushed and treated with 650 i.u. P.M.S. on the twelfth or thirteenth days of the cycle.

Oestrous cycle lengths were of significantly shorter duration in flushed than in unflushed ewes (0.5 day) and in P.M.S.-treated than in untreated animals (0.7 day). Conception rates were of a normal order in all four groups. The mean ovulation rates of those that conceived at first fertile service were respectively 1.33, 1.61, 1.80 and 1.88. The number of ovulations ranged from 1–2 in the unflushed–untreated group and from 1–3 in each of the other three groups.

In the third experiment results obtained from groups of ewes treated with 650 i.u. P.M.S. on the tenth, eleventh, twelfth, thirteenth and fourteenth days respectively, were compared. Mean ovulation rates were approximately similar in all five groups, but (as was also the case in the second experiment) of a lower order than might have been expected from the dose response curve established in the first experiment. Conception rates were satisfactory after injection on the twelfth, thirteenth or fourteenth days, but unsatisfactory after treatment on the eleventh day, and still less satisfactory after treatment on the tenth day.

From the results obtained in these three experiments the following general conclusions were drawn:

(1) Breed or strain differences may be of considerable importance in governing the extent of the ovulation response following P.M.S. treatment.

(2) The magnitude of the ovulation response to P.M.S. treatment depends to some extent on the ‘batch’ of P.M.S. used and/or the strain of ewes employed.

(3) The nutritive status of ewes during the breeding season is not an important factor in determining the ovulation rate following P.M.S. treatment.

(4) When P.M.S. is administered 5 days or less before the onset of the subsequent oestrus, good conception rates are obtained.

(5) Extremely poor conception rates result where animals are treated more than 5 days before oestrus.

(6) Best ovulation and conception rates are obtained when the treatment-to-oestrus interval is 2–3 days.

(7) For optimum results, treatment on the fourteenth day of the cycle is recommended. However, good results are also obtained after treatment on either the twelfth or the thirteenth days. Treatment on the fifteenth day should also be reasonably satisfactory.

(8) When ovulated in similar numbers, and provided conception occurs, ova produced as a result of P.M.S. stimulation are no more prone to fail to undergo normal embryonic development than are naturally ripened eggs.