Abstract: Oxidative stress commonly occurs following heat stress in tropical regions and affects dairy animals. Glutathione protects cells from oxidative damages. This study was carried out to investigate the serum glutathione level in the ram with a fluorometric method and to determine its changes during heat stress condition. Eight mature rams were selected and kept in the same conditions. The rams were maintained during temperate and warm seasons to compare serum glutathione levels during normal and heat stress conditions, respectively. Heat stress was considered when temperature-humidity index was above 84. Serum samples were obtained at 0, 14 and 28 days after beginning of the study during the seasons. Reduced and oxidized glutathione concentrations were determined using a fluorometric assay. The serum concentrations of reduced glutathione in the normal and heat stress conditions were lower than oxidized glutathione. The reduced and oxidized glutathione levels and their ratios were not different between seasons and at different sampling times, although they were significantly changed during sampling times in the normal and heat stress conditions. Present results represent that glutathione levels change during different environmental conditions. It seems that antioxidant defense system was changed to adapt and prevent oxidative stress effects, although needs further researches.