It has been reported that endogenous opioid peptides play a role in the control of the reproductive function. The goal of this study was to evaluate changs in the serum levels of ${eta}$-endorphin during hyperstimulated menstrual cycle and their relationship to serum prolactin levels. Serum ${eta}$-endorphin and prolactin levels were measured during menstrual cycles of 10 normal cylic women hyperstimulated by human menopausal gonadotropin (HMG) and of 10 women by clomiphene/HMG among in vitro fertilization candidates. The results were summarized as follows. 1. In clomiphene/HMG hyperstimulated menstrual cycle the mean serum ${eta}$-endorphin level insignificantly on 2 day before aspiration of oocyte compared to basal level and reached maximum level on 1 day after aspiration. 2. There was a significant peak of the mean serum ${eta}$-endorphin level on 1 day before aspiration in HMG hyperstimulated menstrual cycle. 3. On the same day from aspiration, there was no significant differences in the mean serum ${eta}$-endorphin levels between HMG and clomiphene/HMG hyperstimulated cycles. 4. No significant correlation was noted between serum ${eta}$-endorphin levels and prolactin levels.