Dogs and cats are the most common companion animals in urban environments. Some dogs and cats live in stable home settings, receiving regular care. However, stray dogs and cats are exposed to chronic stress due to life on the streets or in shelters. Stress is correlated with oxidative stress. The dysregulation of redox balance can lead to the accumulation of reactive oxygen species, which promote cellular and molecular damage. In this study, the blood samples were collected from 150 female animals (90 dogs and 60 cats) to isolate peripheral blood mononuclear cells (PBMCs). Total RNA was extracted from PBMCs and subjected to reverse transcription. The expression levels of SOD1 and CAT were quantified using quantitative real-time polymerase chain reaction. Compared with those in stray animals, the SOD1 expression levels were significantly higher (P < 0.01), and the CAT expression levels were non-significantly lower in owned animals. These findings suggest that the expression of antioxidant defence genes varied between owned and stray animals. Thus, oxidative stress regulation is dependent on environmental conditions and lifestyle in companion species.