Abstract. Dynamic changes in an organism of heifers at the end of pregnancy, and in primiparous cows in early lactation, may imply changes in morphological parameters. Their evaluation may contribute to a better understanding of changes, occurring in an organism of pregnant and lactating young cows. Hence, the aim of the study was to investigate, analyze and evaluate the changes in hematological parameters of peripheral blood (RBC, MCV, HGB, MCH, MCHC, WBC, PLT and HCT) in pregnant heifers/primiparous cows, of Polish Holstein-Friesian of Black-and-White variety, during the last trimester of pregnancy and in the first two months of lactation, with particular emphasis on transition period. Blood parameters were analyzed with the use of analyzer: scil Vet abc Plus+ (HORIBA Medical), on the following dates: 90, 30, 14 and 7 days before parturition and 7, 14, 30 and 60 days after parturition. Results of the study indicate that the most abrupt changes were observed for hematocrit, erythrocyte count, leukocytes, thrombocytes and hemoglobin concentration in blood. Parameters of red blood cells, at the end of pregnancy and first two months of lactation, didn’t exhibit any statistically significant changes. Noteworthy, is the convergence of trends in hematocrit and hemoglobin during analyzed period. High and stable hematocrit at the end of pregnancy was due to the stable erythrocyte count in this period. Decrease in RBCs after calving, and lower values during first two months of lactation, caused postpartum decrease in hematocrit and its stabilization at a lower level. These observations denote indirectly changes in blood volume, during analyzed period, what is supported by: tendencies observed for hemoglobin concentration, and stable MCV and MCHC. Obtained results suggest high efficiency of homeostatic mechanisms in pregnant and lactating heifers/primiparous cows. Our results may be useful for comparative studies both in physiology and health disturbances research in cows during the pre and post-partum period.