Objective To investigate the mechanism by which 4-week high-intensity interval training (HIIT ) regulates the mitochondrial unfolded protein response ( UPRmt ) in skeletal muscle and improves mitochondrial function in a rat model of chronic unpredictable mild stress (CUMS). Methods Male SPF-grade SD rats (6~8 weeks old) were divided randomly into control (C), model (M), HIIT+control (HC), and HIIT+model (HM) groups. Rats in the M and HM groups were subjected to CUMS for 8 weeks to establish a depression model,while rats in the HC and HM groups received HIIT for 5 d a week for 4 weeks. The exercise regimen consisted of 3 min high-speed (85% ~90% Smax) combined with 1 min low-speed (50% ~ 55% Smax) uninterrupted repetitive training(Smax is maximum training speed). Behavioral changes were evaluated at weeks 4 and 8. Tissue samples were taken 24 h after the last behavioral test and skeletal muscle mitochondria were examined by transmission electron microscopy. The ATP and reactive oxygen species (ROS) contents were measured by enzyme-linked immunosorbent assays and protein expression levels of activating transcription factor (ATF) 4, ATF5, C/ EBP homologous protein (CHOP), and heat shock protein 60 ( HSP60) were detected by Western blot. Results ( 1) The body mass,number of crossing grids, number of upright positions, sugar-water preference rate, and ATP content were significantly decreased in group M compared with group C (P<0. 01), while the number of damaged mitochondria,ROS content, ATF4, ATF5, CHOP, and HSP60 protein expression were significantly increased (P<0. 01). (2)After 4-weeks of HIIT intervention, the ATP content and ATF4 and ATF5 protein expression levels were significantly increased in the HC group compared with C group (P<0. 05, P<0. 01). The number of crossing grids, number of upright positions, sugar-water preference rate, ATP content, and ATF4 protein expression were significantly increased in the HM group compared with M group(P<0. 01), while the number of damaged mitochondria, ROS content, and ATF5, CHOP, and HSP60 protein expression levels were significantly decreased (P<0. 01). (3)After 4 weeks of HIIT intervention, the number of crossing grids in CUMS rats was significantly positively correlated with ATF4 protein expression, and ROS content was correlated with CHOP protein expression, number of damaged mitochondria, and ATF5 protein expression ( |r| >0. 75, P<0. 01; |r| >0. 75, P<0. 05). Upright frequency was significantly negatively correlated with ATF5 and HSP60 protein expression, the number of crossing grids, the sugar-water preference rate, and the expression of CHOP and HSP60 proteins ( |r| <0. 75, P<0. 05). Conclusions 4-week HIIT intervention can improve mitochondrial dysfunction and alleviate depressive-like behavior in CUMS rats by regulating skeletal muscle UPRmt.