The aim of the study was an evaluation of selected biochemical parameters level in blood of cows of various performance and housed in different environmental conditions. Sixty clinically healthy animals were selected for the study, 20 from each farm of differentiated milk yield (farm A and B), and the same number from individual farms. The cows selected for the study were between 40^th and 60^th day of lactation. Blood serum analysis included: β-hydroxybutyric acid, free fatty acids (FFA), aspartate aminotransferase (AST), γ-glutamyltransferase (GGT), total bilirubin, total proteins and albumins. Cows yield on farm A in the examined period was twice higher compared to farm B. An intensified lipolysis, without an excessive ketogenesis, was noted in high-yielding cows in lactation peak. In low-yielding cows these changes were not significant. An increase in AST and GGT was observed in high-yielding cows in an initial lactation period. Protein level in all sites exceeded the proper values, while albumins level was considerably lowered. On farm A, despite high yield, the aberrances from proper values were small. Regular blood analysis in the herd allow early detection of subclinical states, as well as an assessment of nutrition correctness.

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