This study was designed to measure the changes that occur in luteinizing hormone (LH), follicle-stimulating hormone (FSH), prolactin, testosterone, and cortisol levels in women at the immediate conclusion of a marathon and to compare the results with those in women on the United States Women's Water Polo Team at the conclusion of a strenuous practice session. Thirty-one women runners were included in this study, and each woman served as her own control. Thirteen women were studied from the United States Women's Water Polo Team. In the postmarathon group, cortisol levels showed a mean increase of 211% (p = less than 0.005), FSH levels were unchanged; LH levels, contrary to other published reports, were reduced by 36% (p = less than 0.005); and prolactin levels showed a mean increase of 327% (p = less than 0.005). These results were analyzed in the age groups 20 to 30, 30 to 40, and 41 +. All age groups had similar changes. In members of the United States Women's Water Polo Team, there were no significant changes in cortisol, FSH, LH, or testosterone levels. Prolactin levels showed an average increase of 70% (p = less than 0.002). These data indicate that acute exercise by running a marathon does alter the immediate hormonal profile. The data also indicate that acute exercise in an aquatic program results in a different response.