The influence of melatonin on age dynamics of neutrophils and lymphocytes in blood of laboratory rats kept under different light conditions and predatory mammals--farmer silver fox, raccoon dog kept under natural light (NL) was investigated. The decrease of lymphocyte level, increase neutrophils content and alteration of neutrophils/lymphocytes ratio of aged rats (24 months) kept under natural light (NL) and standard light (LD) and adult silver foxes (2-5 years) kept under natural light in comparison with juvenile animals were detected. The reduced lymphocytes level and increased neutrophil level in rats under constant light (LL) were revealed in younger ages. The melatonin effect was detected in aged rats and adult silver foxes and not observed in juvenile animals.