Plasma beta-endorphin (beta-EP), beta-lipotropin (beta-LPH), and cortisol concentrations were measured by perimenstrual period in 11 patients affected by premenstrual syndrome (PMS) and in 8 asymptomatic healthy volunteers. Blood samples were collected every 2 to 3 days, for 1 month, starting from midcycle. The Menstrual Distress Questionnaire (MDQ) was administered during the testing period. Plasma beta-LPH and cortisol levels remain stable during the perimenstrual period, in both controls and PMS patients. On the contrary, PMS patients showed a decrease of plasma beta-EP in the week preceding menses and during the first days of menstrual flow. Beta-EP values of PMS patients regain normal levels during the next follicular phase. No changes of beta-EP levels were recorded in asymptomatic women. MDQ scores revealed that PMS patients complained of water retention, pain discomfort, and mood swings. The transient and reversible decrease of plasma beta-EP in PMS patients near to and at menses remains to be clarified.