Four experiments dealing with attempts to induce pregnancy in the anoestrous ewe during the summer of 1948 are described. While little has been accomplished towards a practical solution of the problem of regularly inducing such pregnancies, some light has been thrown on reasons for past failures and erratic results. The following conclusions are drawn:

1. In the absence of a spontaneous corpus luteum, ovulation without heat may be induced in anoestrus by 800 i.u. PMS.

2. In the presence of a waning corpus luteum ovulation with heat may be induced in anoestrus by 800 i.u. PMS.

3. In the presence of an active corpus luteum neither ovulation nor heat is induced in anoestrus by 800 i.u. PMS. Occasionally, however, heat may occur without ovulation and with the formation of grossly cystic ovaries.

4. In mid-anoestrus ovulation is not regularly induced by 800 i.u. PMS, due presumably to very low pituitary and ovarian activity.

5. Anoestrus in the ewe is a relative rather than an absolute quiescence, and it is suggested that there is a rhythm of pituitary activity roughly inversely related to the daylight/dark rhythm, deep anoestrus occurring about the time of the longest day. Thus at different stages of anoestrus, different response to the same treatment may be expected in accordance with the proportion of animals falling into the categories 1–4 above.

6. Artificially formed corpora lutea are apparently perfectly normal as regards size, colour and function.

7. Treatment with PMS alone is sufficient to cause follicular maturation, ovulation and corpus luteum formation. There is some evidence, however, that the anoestrous ewe has some difficulty in ovulating all the follicles matured. The subcutaneous use of Prolan does not alleviate this difficulty, but injection of stilboestrol at about the time ovulation is due, may have some effect.

8. The mean ovulation rate of ewes ovulating after 800 i.u. PMS is usually rather higher than the normal for the breed and many cases of definite multiple ovulation have been observed. For this to occur the presence of an old corpus luteum is not necessary. A maximum of six ova have been shed.

9. In mid-anoestrus Welsh ewes show a higher resistance to PMS action than do Suffolks, both as regards numbers ovulating and ovulation rates of those responding.

10. Injection of PMS in increasing divided doses is no more effective in inducing heat than is a single injection. Ovulation without heat occurs as soon as a certain threshold of stimulation (probably about 400 i.u.) is reached.

11. Previous findings have been confirmed that, unless modified by the presence of a spontaneous corpus luteum, ovulation will normally occur within 48 hr. of injection.

12. When 35 mg. testosterone propionate is administered 24 hr. before 800 i.u. PMS, heat, usually without ovulation, will occur in a high proportion of ewes. Where ovulation does occur it rarely coincides with heat.

13. When testosterone propionate is injected with or after PMS the results are much more variable, depending upon whether or not it is absorbed in time to play a role.

14. Testosterone propionate, while inhibiting ovulation, does not inhibit follicular development.

15. Testosterone propionate has no oestrogenic effect. Its action in inducing oestrus in conjunction with PMS has been discussed.

16. The low percentage recovery of fertilized ova following service is attributed to faulty timing between service and ovulation in some cases. In others it may be due to either the ova being potentially unfertilizable or to the uterine and tubal environments being unsuitable for transport of sperm and fertilization. The latter view is favoured.

17. When heat and ovulation occur as a result of PMS injection in the presence of a waning corpus luteum, normal service, sperm transport and fertilization occur.