Plasma beta-endorphin and the menstrual cycle.

beta-Endorphin (beta-E) takes part in the regulation of luteinizing hormone (LH) and prolactin secretion, but little is known about its role in the menstrual cycle. Using a specific assay, plasma concentration of beta-E was followed in healthy women in the periovulatory period and before and during menstruation. The mean concentration of beta-E in the eight women studied showed an increase at the midcycle, being 3.2 +/- 0.5 (standard error) pmol/l on day LH -1 and rising to 4.6 +/- 0.7 pmol/l on day LH 0 and further to 7.5 +/- 1.2 pmol/l on day LH +1. Thereafter, the mean beta-E level remained at 5.3 +/- 0.8 pmol/l and 5.2 +/- 0.4 pmol/l on days LH +2 and LH +3, respectively. Ten additional women were studied before and during menstruation. The highest mean plasma concentration of beta-E, 5.1 +/- 0.9 pmol/l, was found at the onset of menstruation, declining to 3.3 +/- 0.6 pmol/l and 2.9 +/- 0.6 pmol/l on the second and fifth days of menstruation, respectively. These findings suggest that beta-E secretion increases at the time of ovulation. At the onset of menstruation, pain may increase endorphin secretion.