The adaptation of chickens to free-range rearing systems mainly involves the locomotory behavior, which is very different in fast-growing (FG) and slow-growing (SG) strains. This study aimed to compare the effect of moderate locomotory activity (induced and prolonged) on the blood oxidative status in a slow-growing chicken strain with that in a fast-growing one. Thirty FG (Ross 308) birds and 30 SG (Hubbard) birds were divided into 2 groups for each strain and subjected to different treatments: no exercise (the control group [C]) and 1 h of walking at 4 km/h (the exercise group [E]). Daily exercise was promoted by operators, who walked behind the animals around a paddock. Blood samples were obtained weekly from both groups. For the E group, samples were obtained before exercise (E1) and after exercise (E2). Oxidative markers (creatine kinase [CK], reactive oxygen molecular substance [ROMS], thiobarbituric acid reactive substances [TBARS]), and antioxidant compounds (α-tocopherol [α-T], γ-tocopherol [γ-T], δ-tocopherol [δ-T], α-tocotrienol [α-T3], γ-tocotrienol [γ-T3], retinol, and carotenoids) were evaluated. In both strains, the CK level was higher in chickens subjected to exercise; however, its increase was greater in the FG group than in the SG one (1.56-fold vs. 1.08-fold). The antioxidant status was worse in FG strain birds subjected to exercise, whereas the status remained nearly the same in the SG strain birds. The α-T and retinol concentrations were significantly reduced by exercise, primarily in the FG group, whereas the other antioxidant compounds (α-T3, γ-T3, γ-T, δ-T, lutein, and zeaxanthin) were unaffected by strain or treatment. The FG and SG strains had different responses to exercise, and only the SG showed a progressive reduction in TBARS and ROMS values during the 28-day experiment. Accordingly, moderate exercise may be beneficial only when the birds have suitable behavioral characteristics (e.g., higher kinetic activity, rusticity, and explorative nature) or physical characteristics (e.g., low body weight); otherwise, exercise is highly stressful and affects physiology and well-being.