Neutrophil extracellular traps (NETs) are DNA complexes carrying nuclear and cytoplasmic proteins, such as elastase (ELA), cathepsin-G (CAT) and myeloperoxidase (MPO). Mare endometrosis is a chronic degenerative process characterized by excessive collagen in endometrium. While NETs fight bacteria that cause endometritis, they may trigger endometrial fibrogenesis. The aim was to evaluate the in vitro effect of some NETs components on mare endometrial fibrogenesis and determine its relationship with histopathology or estrous cycle. Endometrial explants were incubated with NETs components (ELA, CAT, MPO or oxytocin). Collagen type I (COL1) protein and type I and III (COL3) gene transcription were evaluated in follicular and mid-luteal phases endometria (Kenney and Doig type I/IIA and IIB/III). Increased COL1 occurred with all NETs proteins, although endometrial response to each NETs protease depended on estrous cycle and/or endometrial category. Since ELA enhanced COL1 production, NETs persistence might be linked to endometrosis. Estrous cycle influenced COL1 protein concentration and COL3 transcripts, suggesting that follicular phase may favor endometrial collagen production. However, luteal phase endometria with moderate or severe lesions may be also susceptible to fibrotic effects of NETs constituents. These data propose that NETs involvement in chronic endometritis in mares may act as putative endometrial fibrogenic mediators.