In this study, we examined changes in hormonal status, oxidative status, antioxidant defense, and peripheral blood leukocyte telomere length in male Javanese macaques (Macaca fascicularis) following transfer from group housing to individual cages as a model of social isolation. The study was performed in ten male Javanese macaques aged 11-17 years using a longitudinal design: the parameters were assessed in the same animals before transfer to individual cages and after 60 days of isolation. Malondialdehyde (MDA) concentration, superoxide dismutase (SOD) and catalase activities, cortisol, testosterone, and thyroglobulin levels, as well as absolute mean telomere length, were measured. A statistically significant decrease in telomere length (p = 0.0062), MDA concentration, and catalase activity (p