Growth, mammary development, serum concentrations of growth hormone, prolactin and estradiol which affect the growth and mammary development, were investigated in Korean Native heifers and Holstein heifers from 5 months old to 15 months old. From 5 months old to 15 months old, body weights in Korean Native heifers averaged 37%, lower than those in Holstein heifers of the same age (P<.01). Teat volumes in Korean Native heifers are average 67% smaller than those in Holstein heifers. Analysis of serum protein by secondary electrophoresis showed differences in protein pattern between the breeds and between the individuals. From 5 months old to 15 months old, serum concentration of growth hormone in Korean Native heifers was averge 47% lower than that in Holstein (P<.01), yet prolactin concentrations in serum were about the same between the two breeds. However serum concentration of estradiol in Korean Native heifers was average 20% higher than that in Holstein of same age (P<.01). This result showed evidently that the decrease of the growth hormone concentration in serum, while growing up, causes to retard the growth and mammary development in Korean Native heifers compared to Holstein heifers.