The effects of selection on body weight (BW), dry matter intake (DMI), milk yield, fat percent, and on plasma concentration of insulin (INS), growth hormone (GH), and Insulin-like Growth Factor I (IGF-I) were studied in two groups of first lactation Holstein cows of differing genetic merit (selection vs control). Dry matter intake (DMI) was measured at 45, 90, 180, 225, 270, and 315 d postpartum. Serial blood samples were collected at 30 d intervals for a 5 hr period at 15 min intervals. Selection group cows were heavier (532 vs 514 kg, N.S.) than control cows with a group by days in milk (DIM) interaction (P<.01). Significant differences (P < .05) in energy intake occurred with an average of 25.6 Mcal/d for the control group and 28.6 Mcal/d for the selection group cows. Milk yield and mean milk fat were greater (P<.01) in the selection group cows. The mean estimated production efficiency (kg milk/Meal intake) was .84 and 1.02 (P < .05) for the control and selection group, respectively. Plasma GH was higher (P<.01) and IGF-I was lower (P<.1) in selection group cows compared to control cows. Mean plasma INS concentrations were 821 vs 763 pg/ml (N.S.) for control and selection group cows. A significant (P< .01) interaction occurred between group and month of lactation for GH. The mean IGF-I plasma levels were 170 ng/ml and 139 ng/ml (P < .1) for the control versus the selection cows respectively. The results indicate that selection for milk yield resulted in differences in DMI, milk fat and plasma concentrations of GH and IGF-I. Selection also resulted in increased estimates of production efficiency. In a follow up study the effects of selection on BW, DMI, milk yield, fat percent and response to Growth Hormone Releasing Factor (GRF) as well as glucose infusion were studied in early lactation control and selection group cows. Dry matter intake was measured at 45 and 90 DPP. Serial blood samples were collected at 15 d intervals for a period of either 6 or 15 hr at various intervals. Selection group cows were similar in body weight (494 vs 489, N.S.) than control cows, however energy intake tended to be greater for selection group cows than for control animals (23.3 vs 20.4 Mcal/d, P< .1). Milk yield was greater in selection group cows (P < .01 ). The mean estimated production efficiency was 1.31 vs 1.18 (kg milk/Meal intake) (N.S.) for the selection and control group, respectively. Mean plasma GH was higher on all test days (30, 60, 90, 120 DPP) for selection compared to control group cows (15.6 vs 23.5 ng/ml, P < .01). Mean plasma INS concentrations were 1017 vs 1032 pg/ml (N.S.) for the control and selection groups, respectively, following glucose infusion(.lmg/kg BW). Mean plasma IGF-I concentrations tended to be greater (P<.14) in control group cows compared to selection group cows. No increase in plasma IGF-I was observed in the thirteen hours following GRF (.2ug/kg BW) administration in either group of cows. The results indicate that selection for milk yield resulted in difference in DMI and plasma concentration of GH in response to GRF infusion but it did not affect plasma INS. / M.S.