Pu, Xiaoyan, Xue Lin, Xianglan Duan, Junjie Wang, Jun Shang, Haixia Yun, and Zhi Chen. Oxidative and endoplasmic reticulum stress responses to chronic high-altitude exposure during the development of high-altitude pulmonary hypertension. High Alt Med Biol. 21:378-387, 2020. Objectives: To investigate the effect of endoplasmic reticulum (ER) stress during the development of high-altitude pulmonary hypertension (HAPH) after chronic high-altitude exposure, as well as the association between oxidative stress and ER stress. Methods: Forty male Sprague-Dawley rats were placed in a low-pressure chamber with a simulated altitude of 4,200 m for 0-28 days. Rats were chosen at random on days 0, 7, 14, and 28 of chronic high-altitude exposure and were examined for pulmonary arterial pressure, oxidative stress, apoptosis, and ER stress. Results: Chronic high-altitude exposure caused a continuous deterioration of pulmonary hypertension, which was accompanied by obvious apoptosis of alveolar epithelial cells and remodeling of pulmonary vessels. From day 7 of high-altitude exposure, although the activities of glutathione peroxidase and superoxide dismutase were gradually decreased, the generation of both malondialdehyde and reactive oxygen species was increased in a time-dependent manner. The protein expression of ER stress-related GRP78, PERK, IRE1α, ATF6, ATF4, CHOP, and caspase-12 in lung tissue was significantly upregulated from day 14 of high-altitude exposure. Further, the expression of caspase-12 in alveolar epithelial cells and vascular smooth muscle cells was also increased from day 14 of high-altitude exposure. Conclusions: Early high-altitude exposure first activates oxidative stress; then, it gradually activates ER stress. The activation of ER stress might promote the apoptosis of alveolar epithelial cells and the remodeling of pulmonary vessels by exacerbating the oxidative stress response during the development of HAPH after chronic high-altitude exposure.