Sodium selenite attenuates the chronic heat stress-induced cell viability decreases and ER stress in mouse granulosa cells. Cells were treated with different concentrations of sodium selenite (1, 3, 5, and 7 ng/mL) at 37 °C (A) or at 39 °C (B) for 24 h, and then harvested for analyzing the cell viability by CCK-8 assay. Caspase-3 activity was analyzed using a Caspase 3 Activity Assay Kit (C). Western blot analysis of apoptosis-related protein BAX, ER stress activation marker GRP78 and CHOP are shown (D). The relative protein expression of BAX (E), GRP78 (F) and CHOP (G) were normalized to β-actin. The results of data analysis are shown as the bar graph. The data are presented as mean ± SEM of three independent experiments, and each independent experiment includes three technical replicates. Bars with different lowercase letters are significantly different (p < 0.05).