INTRODUCTION: Wildland firefighters (WLFF) are confronted with numerous physical and mental stressors. Pre-fire season includes an intense two-week critical training period; a preparatory phase that sometimes results in injuries, illness, and rhabdomyolysis. PURPOSE: To identify physiologic changes to oxidative stress and other metabolic biomarkers that occur during 2 weeks of critical training in WLFF. METHODS: Eighteen male (29.4±1.1 yr, 182.1±1.6 cm) and three female (26.7±2.6 yr, 169.5±4.2 cm) participants were recruited from a Type I interagency hotshot fire crew and monitored for the duration of their two-week critical training. Subjects were asked to arrive fasted and uncaffeinated for blood draws on days 1, 4, 8, and 11. Plasma was analyzed for changes in the metabolic profile and oxidative stress markers 3-Nitrotyrosine (3-NT) and 8-Isoprostane (8-ISO). A one-way repeated measures ANOVA was used to analyze 8-ISO and 3-NT. Paired samples t-tests were used to compare metabolic biomarkers. Data presented as mean±SEM. RESULTS: Critical training elicited a decrease in total cholesterol (173.6±12.1 to 153.4±8.6 mg·dL-1, p=0.01), hemoglobin A1c (5.2±0.1 to 5.1±0.1 %, p=0.003), hemoglobin (15.5±0.4 to 14.3±0.3 g·dL-1, p=0.003), and estimated plasma volume (53.8±0.7 to 50.7±1.4 %, p=0.005) from day 1 to 11. No difference was observed in high-density lipoprotein cholesterol. A main effect for time was observed in 8-ISO (p<0.001) and 3-NT (p=0.033). A significant decrease was observed in 8-ISO at day 4 and 8 when compared to day 1 (day 1: 15.5±1.3, day 4: 11.8±1.0, day 8: 12.9±1.1 pg·mL-1). 3-NT was significantly elevated from day 4 to day 8 (day 4: 2.4±0.6, day 8: 2.9±0.6 μg·mL-1). CONCLUSION: These data suggest that the exertion required of WLFF during critical training results in positive alterations to the metabolic profile. The unexpected decreases in oxidative stress markers may reflect adaptation to the stressors of critical training, although this cannot be confirmed. Additional markers should be examined across other WLFF crews to confirm and provide further information on alterations during this two-week period. Funded by the US Forest Service 16-CR-11138200-005.