AIMS: The role of the parasympathetic nervous system (PNS) in the pathogenesis of nonalcoholic steatohepatitis (NASH) is largely unknown. In this study, the effect of PNS modulation on NASH was investigated using chemogenetics. MAIN METHODS: A streptozotocin (STZ) and high-fat diet (HFD)-induced NASH mouse model was used. To activate or inhibit the PNS, chemogenetic human M3-muscarinic receptor coupled with either Gq or Gi protein-containing viruses was injected into the dorsal motor nucleus of the vagus at week 4 and clozapine N-oxide was administered intraperitoneally for a week from week 11. Three groups (PNS-stimulation, PNS-inhibition, and control) were compared in terms of heart rate variability (HRV), histological lipid droplet area, nonalcoholic fatty liver disease activity score (NAS), the area of F4/80-positive macrophages, and biochemical responses. KEY FINDINGS: The STZ/HFD-treated mouse model showed typical histological characteristics of NASH. HRV analysis confirmed that PNS-stimulation and PNS-inhibition groups had significantly higher and lower PNS activity, respectively (both P < 0.05). The PNS-stimulation group showed a significantly smaller hepatic lipid droplet area (14.3 % vs. 20.6 %, P = 0.02) and lower NAS (5.2 vs. 6.3, P = 0.047) than the control group. The area of F4/80-positive macrophages was significantly smaller in the PNS-stimulation group than in the control group (4.1 % vs. 5.6 %, P = 0.04). The PNS-stimulation group showed a lower serum aspartate aminotransferase level than the control group (119.0 vs. 356.0 U/L, P = 0.04). SIGNIFICANCE: In STZ/HFD-treated mice, chemogenetic stimulation of the PNS significantly reduced hepatic fat accumulation and inflammation. The hepatic PNS may play a pivotal role in the pathogenesis of NASH.