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.