This study was conducted to investigate individual metabolic and endocrine adaptation to lactation under conditions of identical housing and feeding conditions in high-yielding dairy cows. Forty-five cows were studied on a research farm under standardized but practical conditions. From wk 2 before calving until wk 14 postpartum, blood samples were collected at weekly intervals and assayed for blood chemistry and various metabolites and hormones. Body weight, BCS, and backfat thickness were also recorded weekly. Milk yield, milk composition, and feed intake and energy balance were accordingly measured during the postpartum phase. The animals were retrospectively classified according to their plasma concentration of beta-hydroxybutyrate (BHB): cows in which a BHB threshold of 1 mM was exceeded at least once during the experiment were classified as BHB positive (BHB+); cows with BHB values consistently below this threshold were classified as BHB negative (BHB -). Using this classification, differences for NEFA and glucose concentrations were observed, but the mean calculated energy balance did not differ between the groups during the experimental period (-22.2 MJ of NE(1)/d +/- 4.7 for BHB+ and -18.9 MJ of NE(1)/d +/- 4.9 for BHB-). In BHB+ cows, the peripartum decrease (P < 0.05) of BW, BCS, and backfat thickness was more pronounced than in BHB- cows. Mean milk yields did not differ between groups. However, BHB+ cows had greater milk fat and lesser milk protein contents (P < 0.05), resulting in a greater (P < 0.05) fat:protein ratio than in BHB- cows. Thus, to some extent, cows were able to compensate for the negative energy balance by adjustments in performance. Milk acetone concentrations followed BHB concentrations in blood. Insulin-like growth factor-I and leptin concentrations were greater (P < 0.05) in BHB- cows during the time of observation than in the BHB+ cows. Comparing the reproductive variables recorded (first increase of progesterone, first service conception rate, number of services per conception, interval from calving to first AI, interval from first AI to conception, and days open) between the 2 groups yielded no significant differences. Our findings imply that despite comparable energy balance, there is considerable individual variation of the adaptive ability of cows during early lactation based on a variety of metabolic and endocrine variables.