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Abstract The objective was to evaluate the supplementation effect of selenium (Se) and DL-tocopherol (E) on blood lactate (LAC), cortisol (COR), and malondialdehyde (MDA), in horses moderately exercised in a polluted environment. Police horses of Mexico City were arranged in a balanced factorial trial (2 × 2) with repeated measures. Low (L) and high (H) levels of Se and E supplementation were evaluated (LSe, 0.1; HSe, 0.3 mg Se/kg dry matter; LE, 1.6; HE, 2.0 IU vitamin E/kg body weight). Experimental groups were LSeLE, LSeHE, HSeLE, and HSeHE. The study that lasted 77 days was distributed as follows: adaptation (28 days), exercise (28 days, three consecutive days of moderate exercise were alternated with four days of inactivity), rest (7 days), and recovery without supplementation for readjustment to police handling (14 days). Once a week (11 week), blood samples for LAC, COR, MDA, blood selenium (B-Se), and α-tocopherol (TOC) quantifications were taken. Data were analyzed with a model for fixed (Se, E, day, and their interactions) and random (the horse nested within the treatment) effects. The relationships between Mexico City's criteria air pollutants (CAPs) concentrations and blood analytes were analyzed by multiple sequential regression analysis. Concentrations of all blood analytes were affected (P <.05) by day while Se affected (P <.05) COR, B-Se, and TOC. No analyte was affected by E (P >.05). Relationships were observed between LAC, COR, MDA, CAP, and day (P <.05). It is concluded that supplementation with high selenium levels could benefit horses exercising in polluted environments. Further research on chemical forms and feeding doses of both Se and E is needed. Graphical abstract Highlights • Serum cortisol (COR) concentrations are affected by oral antioxidant supplementation. • Moderate exercise increases blood: lactate (LAC), COR, and malondialdehyde concentrations. • In horses, LAC and COR blood concentrations are affected by air pollutants. • Dietary recommendations for vitamin E and their nutritional interactions should be reviewed. • Serum COR, pollutants, and exercise relationships are affected by Se supplementation. [ABSTRACT FROM AUTHOR]