PIP: Five normal ovulating women were followed through 7 menstrual cycles. Basal body temperature was recorded each morning at the same time and 24-hour urine specimens were collected from Day 5 to the last day of each menstrual cycle. Urinary epinephrine and norepinephrine were determined. Serum luteinizing hormone (LH) and cervical mucus tests for spinnbarkeit, ferning, and lysozyme helped confirm the day of ovulation. Preovulatory norepinephrine excretion ranged from 7.45 to 18.83 mcg per 24 hours; postovulatory norepinephrine was significantly elevated and ranged from 14.7 to 51.6 mcg per 24 hours. Mean values of epinephrine did not differ before and after ovulation. It is postulated that progesterone causes an increased excretion and production of norepinephrine which is responsible for the increased thermogenesis after ovulation. The basic mechanism for thermogenesis is norepinephrine and not progesterone.