The purpose of the present study was to characterize the progression of red blood cell volume (RBCV) expansion and potential volumetric and endocrine regulators of erythropoiesis during endurance training (ET). Nine healthy untrained volunteers (age=27±4 years) underwent supervised ET consisting of 3-4 x 60 min cycle ergometry sessions per week for 8 weeks. Plasma volume (PV), RBCV and overnight fasting hematological markers were determined prior to and at weeks 2, 4 and 8 of ET. In addition, plasma erythropoietin (EPO), cortisol, copeptin and pro-atrial natriuretic peptide concentrations were measured during a 3-hour morning period at baseline and post-exercise at weeks 1 and 8. PV increased from baseline (2405±335 ml) at weeks 2, 4 and 8 (+374±194, +505±156, +341±160 ml, respectively, P<0.001). Increases in RBCV from baseline (1737±442 ml) were manifest at week 4 (+109±114 ml, P=0.030) and week 8 (+205±109 ml, P=0.001). Overnight fasting plasma EPO concentration increased from baseline (11.3±4.8 mIU•ml^-1) at week 2 (+2.5±2.8 mIU•ml^-1, P=0.027) and returned to baseline concentration at weeks 4 and 8. Higher 3-hour-post-exercise EPO concentration was observed at week 1 (11.6 mIU•ml^-1) compared with week 8 (8.4±3.9 mIU•ml^-1, P=0.009) and baseline (9.0±4.2 mIU•ml^-1, P=0.019). Linear relationships between EPO concentration and hematocrit (β =-56.2, P < 0.001) and cortisol (β=0.037, P<0.001) were detected throughout the ET intervention. In conclusion, ET leads to mild transient increases in circulating EPO concentration concurring with early PV expansion and lowered hematocrit, preceding gradual RBCV enhancement.