Daily s.c. injections of buserelin were commenced in the mid-luteal phase of the preceding cycle in 118 women undergoing in-vitro fertilization (IVF) and embryo transfer. Ovarian and pituitary suppression was said to have been adequately achieved when serum oestradiol was less than 50 pg/ml, serum LH less than 2.0 IU/l, no ovarian cysts greater than or equal to 10 mm diameter were present and menstruation had occurred. Nine groups of women were retrospectively identified after the administration of buserelin for 12 days according to whether pituitary and ovarian suppression had been achieved or not, and the reason for extended buserelin treatment prior to ovarian stimulation. Upon adequate suppression, patients were grouped in terms of the duration of exposure to buserelin, and ovarian stimulation was then started by daily injections of human menopausal gonadotrophin. There appeared to be no differences in the ovarian response for women down-regulated by day 12, 19 or greater than or equal to 26 days; those women requiring extended buserelin treatment did equally well compared to those women down-regulating quickly, in terms of number of oocytes recovered and fertilization rate. Clinical pregnancy rates per embryo transfer were 27/68(40%), 8/33(26%) and 4/17(24%) for those women down-regulated by days 12, 19 or greater than or equal to 26 respectively, and were not significantly different.