Six months of exercise training was associated with decreased premenstrual symptoms in two groups of women. There was no change in symptoms in nontraining women. Eight sedentary (ST) women increased running from 0 to 76 +/- 26 km/cycle (mean +/- standard deviation) over 6 months and seven runners (MT) trained for a marathon (42.2 km). Six normally active, nontraining (C-NT) women kept their activity constant. Each subject completed monthly intensity-graded questionnaires or kept daily symptoms diaries concerning premenstrual symptoms. All monitored basal body temperature, weight, and exercise. Gonadal steroids were measured in ST women. For ST subjects, breast (P = 0.005), fluid (P = 0.01), and personal stress (P = 0.025) decreased. MT women experienced decreased fluid (P = 0.034) and depression (P = 0.014). Anxiety tended to decrease (P = 0.087). ST and MT subjects experienced decreases in premenstrual symptoms without documented hormonal, menstrual cycle, or weight changes. These symptom changes appear to be the earliest evidence of the effects of conditioning exercise on the reproductive system.