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17β-Oestradiol was administered intramuscularly to twentyeight heifers either before, simultaneously with or after a subcutaneous injection of 3000 i.u. of pregnant mares' serum (pms). This was followed by an intravenous injection of 2000 i.u. ofhuman chorionic gonadotrophin (hcg). The animals were bred and slaughtered, the reproductive tract flushed and the ova examined microscopically.Oestrogen treatment tended to reduce the number of developed follicles when applied before pms, but it significantly increased the number when applied after pms. The proportion of follicles which ovulated was not affected by oestrogen. In a group treated with oestrogen after pms, the number of fertilized ova per cow ranged from six to seventeen with an average of 11·3 which was considered an improvement over superovulated cattle not treated with oestrogen. There was no evidence that the rate of tubal transport of ova was altered by the oestrogen therapy applied.

Embryos were recovered from super-ovulated rabbits and transferred to beef cattle. Three techniques of embryo transfer were attempted: (a) non-surgical, (b) non-surgical with CO2insufflation, using the “Kidde” insufflator and (c) surgical via a paralumbar laparotomy. An apparatus made of three plastic tubes and two long metal needles was used to by-pass the cervix and to transfer the embryos non-surgically into the uterus. Carbon dioxide insufflation through the cervix was essential for successful transfer. This non-surgical technique was inferior to the surgical technique in terms of percent of embryos successfully transferred. Viable rabbit embryos of 16- and 32-cell stages developed to the blastocyst stage in the cow's uterus.Embryos were recovered from super-ovulated cattle and transferred by laparotomy to the Fallopian tubes or uteri of 1-, 2-, 3- and 4-day pseudopregnant rabbits. Viable cattle embryos of 1- to 8-cell stages failed in most cases to cleave when kept for 48 hr. in the genitalia of pseudopregnant rabbits.

Two fertilized eggs, recovered from two gonadotrophintreated Holstein heifers 4 and 6 days after ovulation, were transferred to the uteri of two synchronized recipient heifers by means of non-surgical techniques. A specially designed apparatus was used for the transfer. Both recipients became pregnant and one gave birth to a normal bull calf after 268 days. It was established by blood-typing that the calf had developed from a transferred embryo. The other recipient aborted on the 80th day of pregnancy.