The effect of genetic selection for milk yield on lactation yield, net energy balance (NEB) and on plasma growth hormone (GH), insulin (INS), prolactin (PRL), nonesterified fatty acids (NEFA) and glucose was studied in primiparous Holstein cows. Net energy balance was calculated and serial blood samples were collected at 0, 45, 90, 180 d postpartum (dpp) and 14 d of the dry period over a 7 h period via jugular cannulae. After 2.5 h of blood collection, growth hormone releasing factor (GRF) was administered at 0, 45, 180 dpp and 14 day of the dry period, while epinephrine was administered at 90 dpp. Nonesterified fatty acids were quantified in 90 dpp samples only. Basal and response periods for each hormone and metabolite were compared by analysis of variance. Milk yield was greater (P<.05), NEB was decreased (P<.05) and plasma GH was greater (P<.05) in selection cows (high-yielders) compared to control cows (low-yielders), while PRL, INS, glucose and NEFA were not different. Growth hormone increased in both groups in response to GRF at all days postpartum measured, while PRL, INS and glucose were not altered. Epinephrine administration at 90 dpp, stimulated an increase in plasma NEFA, glucose and INS in both groups. Control cows showed a greater (P<.01) INS response than selection cows. Results indicate differences exist in GH concentration between genetically selected high- and low-yielding Holsteins during early lactation, but the question remains if these differences are due to energy balance differences or differences in genetic merit for milk yield. / Master of Science