Exercise-induced changes in enzymatic O-methylation of catecholestrogens by erythrocytes of eumenorrheic women. Med. Sci. Sports Exerc., Vol. 29, No. 12, pp. 1580-1587, 1997. The present study was designed to assess the effects of acute exercise and short-term intensive training on catechol-O-methyltransferase (COMT) activity. COMT inactivates catecholamines and converts primary catecholestrogens (CE) into their O-methylated form yielding the 2- (2-MeOE) and 4-methoxyestrogens (4-MeOE). Blood samples were obtained from 15 previously untrained eumenorrheic women (mean ± SE, ˙VO2max: 43.8 mL·kg-1·min-1 ± 0.6) before and after a 5-d intensive training period, at rest and during incremental exercise. COMT activity was determined in the erythrocytes (RBC-COMT) after incubation of blood lysate with primary CE. The formation of both 2- and 4-MeOE was significantly higher (P < 0.05) during the luteal (LPh) than during the follicular phase (FPh). The amount of 2-MeOE formed (FPh: 4.2± 0.2%; LPh: 4.9 ± 0.2%) was significantly greater than the produced amount of 4-MeOE (FPh: 1.4 ± 0.1%; LPh: 1.5 ± 0.1%)(P < 0.05). Both before and after training, incremental exercise did not significantly alter RBC-COMT activity although we observed a trend for RBC-COMT activity increasing proportionally with the exercise intensity. After a brief period of exhaustive training, during rest the formation of 2-MeOE(FPh: +16.7%, LPh: +15.7%) and 4-MeOE (FPh: +28.6%; LPh: +40%) was significantly (P < 0.05) increased. The results of the present study are consistent with earlier findings reporting increased plasma concentrations of O-methylated CE following training. It is concluded that RBC-COMT activity is increased by brief intensive training, but not by acute exercise. We speculate that an increase in COMT-catalyzedO- methylated of CE may indicate that less COMT is available to deactivate norepinephrine.