Two experiments were conducted to evaluate the effect of recombinantly derived bovine somatotropin (rbST) on milk production, feed efficiency, milk composition and hormones in lactating dairy cows. In daily injection treatment, 15 Holstein cows were allocated to 3 treatments consisted of 0 ㎎/day, 12.5 ㎎/day and 25.0 ㎎/day. The injection of rbST was conducted intramuscularly once a day beginning at day 28 postpartum and continuing for 210 days. In a sustained release treatment, 12 Holstein cows were allocated to 3 treatments consisted of 0 ㎎/2 weeks, 500 ㎎/2 weeks and 750 ㎎/2 weeks of rbST (SR-rbST) injected subcutaneousely once at 2 week intervals beginning at day 254 postpartum and continuing for 28 days. Cows were fed concentrates containing 19.0% CP and 72.4% TDN, and corn silage (30 ㎏/day) throughout the experimental period. The rbST injection (25 ㎎/day) increased 4.0% FCM milk production by 18.0% during treatment period and by 11.7%, compairing with the milk yield of pretreatment period for control cows, while SR-rbST (750 ㎎/2 weeks) injection by 7.0% above control group. There are slightly decreased tendencies in body weight gain after rbST treatments. Milk fat, protein and lactose percentages were slightly highter in rbST treatment groups (12.5 ㎎/day and 25.0 ㎎/day) but did not show statistical difference. However, DM intake of concentrate was increased by 11.0%. showing the highest feed efficiency (㎏ 4.0% FCM/㎏ DM) in 25,0 ㎎/day treatment. Concentration of bST in serum was significantly (P$lt;0.05) higher in rbST treatments (23. 96 and 49.23 ng/㎖) than that of control (15.26 ng/㎖) but milk bST concentration did not differ among treatments (0.32, 0.35 and 0.36 ng/㎖). Furthermore, serum IGF-1 were increased by two-fold with two rbST injection groups (63.53 and 65.60 ng/㎖) although milk IGF-I concentrations were similar for all treatments. Therefore, it could be concluded that rbST and SR-rbST administrations to lactating Holstein cows efficiently improve dairy performances without altering milk composition in a whole and a late lactation period, respectively.