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Objective: To examine the interaction between circulating beta-endorphin levels and sex steroids during sustained submaximal exercise in runners who are either anovulatory and oligomenorrheic (AO) or ovulatory and eumenorrheic (EO)., Design: Controlled clinical study., Setting: General clinical research center at an academic medical center., Patient(s): Three AO and four EO runners., Intervention(s): The athletes underwent 60 minutes of submaximal treadmill exercise on three separate occasions. Anovulatory and oligomenorrheic runners underwent exercise at baseline and after physiologic estrogen and combined estrogen and progesterone replacement. Ovulatory and eumenorrheic runners underwent exercise in the follicular and luteal phases and after GnRH agonist desensitization., Main Outcome Measure(s): Serum cortisol, beta-endorphin, progesterone, estrogen, and gonadotropin levels at rest and during exercise., Result(s): Serum levels of E2 increased in response to exercise in both EO and AO runners during sex steroid replacement. Baseline peripheral beta-endorphin and cortisol levels were not different between the EO and AO groups. A significant increase in beta-endorphin levels in response to exercise occurred only in the EO group after GnRH agonist desensitization., Conclusion(s): Alterations in menstrual cyclicity and ovulation in conditioned runners probably are not due to an increase in opioid tone. The hypothalamic-gonadotropic axis appears to be intact in AO runners, as measured by the gonadotropic response to exogenous exposure to estrogen and progesterone. Sex steroid administration had no effect on basal beta-endorphin levels, but this probably was not due to preexisting increased opioid tone.