The aim of the research was to determine the effect of the number of rearing kits on selected blood parameters in polar fox females in the fifth week of lactation. The haematological parameters were tested in the blood of polar fox females with small (group A-mean number of pups: 3.5) and large (group B-mean number of pups: 10.7) litter. In female polar foxes rearing 9-13 kits (group B) there was found a lower number of red blood cells, a lower haematocrit value and lower concentration of haemoglobin. No significant (P<0.05) effect of litter size on the number of white blood cells was observed, however there was an essential (P<0.05) increase in the percentage of neutrophiles as well as lowering of lymphocytes in the total count of leucocytes in the female of polar foxes from B group. A large number of pups in litter significantly (P<0.05) lowered the content of glucose in female blood on the 35th day of lactation.