The influence of the different phases of the menstrual cycle on platelet-poor plasma norepinephrine (NE) and serotonin (5HT) was examined in 17 normal volunteers. The examinations were performed consecutively during 3 phases of the ovulatory cycle: 1) follicular phase, 2) ovulation, and 3) luteal phase. This investigation was initiated after a preliminary study in 51 volunteers showed wide and consistent variations of plasma NE and 5HT during the different phases of the cycle. Since in this first group the determinations had not been performed consecutively in the same subjects, and the changes observed in the different phases of the cycle could reflect interpersonal variations, the determinations were performed consecutively in a second group, concomitantly with serum estradiol (E2) and LH measurements. The results showed a decrease in plasma 5HT from the follicular phase [144.3 ± 69.3 nmol/L (± SD)] to ovulation (55.7 ± 41.4; P < 0.001) and a subsequent increase in the luteal phase (141.3 ± 96.4; P < 0.01). The nadir in plasma 5HT showed an inverse correlation with serum LH (r = -0.07). Plasma NE increased from the follicular phase (1226.5 ± 475.1 pmol/L) to ovulation (1694.0 ± 564.4; P = 0.027) and reached a maximum in the luteal phase (2335.0 ± 728.2; P = 0.0034). This rise correlated positively with serum E2. In conclusion, plasma 5HT and NE vary with the different phases of the menstrual cycle. Plasma NE rises during ovulation and seems to to correlate positively with serum E2 levels. Plasma 5HT reaches a nadir during ovulation and correlates inversely with serum LH.