A total of 20 women with regular menstrual cycles normal ovulatory function were randomized to receive 3.75 mg or 1.87 mg of D-Trp-6-luteinizing hormone-releasing hormone (LHRH) in microcapsules either in the early follicular phase (day 2) or during the mid-luteal phase (day 21) of the menstrual cycle. Pituitary--ovarian function was studied by sequential plasma measurements of oestradiol, follicle stimulating hormone, luteinizing hormone and progesterone. Our results show that the full, standard dose (3.75 mg) of D-Trp-6-LHRH depot and the half-dose (1.87 mg) were equally effective in their desensitizing action, and that desensitization occurs more rapidly with early follicular phase administration of the analogue.