2,3-Diphosphoglycerate red cell concentration (2,3-DPG) was significantly higher during the last week of the menstrual cycle of 5 women (1.08 .+-. 0.09 mol .cntdot. molHb-1) (M .+-. 1 SE) than during the 1st part and the last but 1 wk of the cycle (0.86 .+-. 0.04 mol .cntdot. molHb-1, P < 0.01). PACO2 [arterial CO2 tension] was significantly lower during the last week of the cycle (36.3 .+-. 1.3 torr) than during the 1st part and the last but 1 wk of the cycle (40.1 .+-. 0.8 torr, P < 0.001). This hypocapnia was due to transient hyperventilation in the 2nd part of the menstrual cycle. Five men were studied during 4 wk to determine control values. No change was observed. The 2,3-DPG mean value (1.10 .+-. 0.01 mol .cntdot. molHb-1) was not significantly different of the values observed in the women during the last week of the cycle. The 2,3-DPG increase during the last week of the menstrual cycle was probably due to a respiratory alkalosis induced by progesterone secretion.