Six male well trained cross‐country skiers were exposed to 6 d of training 4 h/d at an altitude of 2700 m. Living quarters were at 2000 m. Measurements of serum‐erythropoietin concentration ([EPO]), erythrocyte‐ 2,3 diphosphoglycerate (DPG), haemoglobin (HB), packed cell volume (PCV), body mass (BM) and resting heart rate (HR) were made before, during and after moderate‐altitude training. Total body haemoglobin (THB) was determined before and after altitude training. Plasma volume (PV) was calculated from THB, PCV and HB. The daily EPO production was calculated from [EPO]and PV. Moderate‐altitude EPO production at days 2, 3 and 6 was significantly higher than prealtitude levels. DPG increased significantly at moderate altitude. Pre‐altitude and moderate‐altitude EPO productionvsDPG were correlated. Moderate‐altitude levels of BM decreased, PV increased and HB decreased (NS). During moderate‐altitude training, DPG exerts a certain feedback regulation of EPO production. The temporal courses of [EPO] and EPO production at moderate altidude are not affected by