Background. Severe stress and trauma-related conditions, such as post-traumatic stress disorder (PTSD), are associated with accelerated biological aging; telomerase activity (TA) is a dynamic biomarker that responds faster than telomere length. Methods. Male C57BL/6J mice (9—12 weeks old, n = 24, 12 per group) were used, and a stress-associated phenotype was induced using the single prolonged stress (SPS) protocol, after which animals were left undisturbed for 7 days [1, 2]. Behavioral testing included OF, EZM, FC, and NOR. TA was measured by TRAP-qPCR [3] in bone marrow, testis, prefrontal cortex, hippocampus, and adrenal glands. Normality (Shapiro-Wilk) and group differences (Mann-Whitney U or Student’s t-test) were tested at p < 0.05 for behavioral and TA data. Results. Behavioral tests showed a stress-associated phenotype with enhanced fear and ambiguous anxiety changes. FC and OF freezing increased (p < 0.05), but EZM open zone time contradicts classic anxiety. NOR discrimination index showed no difference between groups. Bone marrow TA showed no difference. Testicular TA increased ~75-fold post-stress (p = 0.0002). This marked increase needs confirmation due to confounding factors. TA in cortex, hippocampus, and adrenals was below detection limits. Conclusions. The stress-associated phenotype was accompanied a tissue-specific testicular TA increase, with no bone marrow alterations. Below-detection-limit TA in brain and adrenals likely reflects assay sensitivity and low basal activity. These findings suggest the reproductive system is an early stress-sensitive target, warranting larger multi-time-point studies.