The Northern bat, Eptesicus nilssonii, is the most common species among northern chiropteran populations. The species is noted for its long hibernation period with low mortality rate. We investigated the morphology of E. nilssonii blood cells during hibernation, hematological parameters including hemoglobin level, red blood cell (RBC) count, total white blood cell (WBC) count, and differential WBC count. The study revealed changes of several hematological parameters in hibernating bats, with some variation during the hibernation season. Hibernating E. nilssonii were found to have a low total WBC count similar to other hibernators; no significant variation in WBC count was noted during hibernation. Differential leukocyte count showed a significant reduction in total monocytes in winter (in the middle of the hibernation period). Total lymphocyte, band neutrophil, and basophil counts increased in spring (termination of the hibernation period).