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Simple Summary: Nowadays, most consumers prefer table eggs from free-range laying hens. At the same time, relatively few studies exist on the possible effects of free-range rearing on the oxidative status of fowls and their egg production. This trial studied the influence of environmental conditions (cold, thermoneutral and hot period) on the oxidative status of different genotypes of free-range laying hens and the effect of oxidative stress (OS) on their egg production. The factor "temperature period", compared to "year" and "genotype", had the most significant influence on all biochemical parameters determining OS. The chicken genotypes showed differences in their susceptibility to OS, and this had an effect on egg production. The OS is genotypically specific and can play a significant role in determining welfare and egg production in free-range systems. This study aimed to assess the changes in the oxidative status of six genotypes of free-range laying hens during cold, thermoneutral, and hot periods by measuring the levels of lipid peroxidation (LPO), total glutathione (tGSH), and the activity of antioxidant enzymes catalase (CAT), superoxide dismutase (SOD), and glutathione peroxidase (GPx) in erythrocyte suspension, in relation with their egg production. Two identical experiments were conducted in two consecutive years. Thermal stress adversely affected the oxidative status of hens. The induced OS is expressed by an increase in LPO and the activities of antioxidant enzymes SOD and GPx during cold and hot periods and a decrease in CAT and tGSH during the cold period in both years. The factor "temperature period", compared to "year" and "genotype", had the most significant influence on all biochemical parameters (p < 0.001). Significant phenotypic correlations (p < 0.05) were detected among studied biochemical parameters, except between SOD and tGSH. The chicken genotypes showed differences in their susceptibility to OS and this had an effect on egg production—from 37.87% to 74.93%. The OS is genotypically specific and can play a significant role in determining welfare and egg production in free-range systems. [ABSTRACT FROM AUTHOR]