We examined the influence of human chorionic gonadotrophin (HCG), used as an ovulation inducer and/or for supporting the luteal phase, on the next ovarian cycle. Four women received 10,000 IU of HCG at mid-cycle and another four received the same dose plus 1500 IU on the 17th, 19th and 21st days of the cycle. In the menstrual cycle prior to our experiments, venous blood samples were collected and vaginal ultrasound of the ovaries was performed every other day from day 21-28; the same data were also collected on days 1-10 of the experiment cycle. In such a way, control values were obtained. After the administration of HCG, venous blood samples were collected and ultrasound was performed in the same way and on the same days as in the controls. Follicle stimulating hormone (FSH) and luteinizing hormone were determined by radioimmunoassay in all blood samples, and HCG only in samples collected after the experiment. The results showed that only FSH was lower in the late luteal phase after the administration of 10,000 IU of HCG. Follicular diameters were higher during the follicular phase than during the previous cycle only in women who received the low dose of HCG. In addition, one woman presented with detectable HCG in the following ovarian cycle. The use of HCG in the preceding cycle may reduce FSH and develop persistent follicles in the subsequent cycle. We suggest that an ultrasound of the ovaries should be performed before starting a new ovulation induction cycle in a woman who has received HCG in the previous cycle.(ABSTRACT TRUNCATED AT 250 WORDS)