The effect of photoperiodic conditions in the northwestern Russia (Republic of Karelia, Petrozavodsk) and the pineal gland hormone melatonin (100 μg / animal) on the composition of the leukocyte formula and morphometric parameters of peripheral blood lymphocytes in females and males of the Syrian hamster (Mesocricetus auratus) was studied. The animals were divided into 2 groups: control (LD: 12 h light / 12 h dark) and experiment (NL: decrease in the duration of the light phase of the day from 19: 36/4: 24 to 12/12, typical for the Republic of Karelia in the period from 25.06 .18 to 25.09.18). Each group was divided into 2 subgroups: hamsters of the 1st subgroup received drinking water without melatonin (LD, NL), the 2nd - at night, melatonin (100 μg / animal) (LDmel, NLmel). As a result of the experiment, it was shown that the northwestern photoperiod only slightly changed most of the parameters of the leukocyte formula in females and males. The effect of the length of daylight hours on the total leukocyte count in females was not revealed, but a decrease in the number of stab neutrophils was found with a reduction in the photoperiod. Both females and males in NLmel showed an increase in the ratio of neutrophils to lymphocytes compared to LD and NL. Morphometric analysis of lymphocytes showed that a decrease in the length of daylight hours and melatonin affect the area of lymphocytes and the nuclear-cytoplasmic ratio (NCR): in females and males in the NL and LDmel groups, the cell area decreased by the end of the experiment, while the NCR decreased only in NL and NLmel females with a decrease in the duration of daylight. The noted changes in the studied parameters can be mediated by the influence of the endocrine glands, in particular the thymus, adrenal glands and gonads, which are involved in the regulation of leukopoiesis and cellular metabolism.