In this study we have examined the correlation between activity of erythropoiesis and serum concentrations of erythropoietin and insulin-like growth factor I in male and female rats during accelerated growth (day 30-90). We found that fractional incorporation of iron into newly formed red blood cells was linearily correlated with body weight gain. Total iron incorporation into newly formed red blood cells reflecting total daily red cell formation increased almost linearily between day 25 and 80 after birth in both sexes. While serum erythropoietin concentrations decreased in the time interval investigated (25-120 days), serum IGF-I levels increased in both sexes between day 25 and 55. In this period, individual values of total iron incorporation into red blood cells and serum IGF-I concentrations were linearily correlated. Our observations support the concept that IGF-I rather than erythropoietin modulates erythropoiesis during accelerated growth and thus manages a proportional increase in body mass and oxygen transport capacity.