Rat GM-CSF (Granulocyte–Macrophage Colony-Stimulating Factor, CSF2) is a hematopoietic cytokine belonging to the colony-stimulating factor family, which also includes G-CSF (CSF3) and M-CSF (CSF1) that regulate the development, proliferation, and activation of myeloid immune cells. In rats (Rattus norvegicus), GM-CSF is produced by activated T lymphocytes, macrophages, endothelial cells, fibroblasts, and epithelial cells in response to infection, inflammatory cytokines, or immune stimulation. Rat GM-CSF binds to the GM-CSF receptor (CSF2R) expressed on hematopoietic progenitor cells and mature myeloid cells, activating intracellular signaling pathways such as JAK/STAT, MAPK/ERK, and PI3K–AKT, which promote the proliferation and differentiation of granulocytes, macrophages, and dendritic cells and enhance their phagocytic and antigen-presenting functions. In rat immunity, GM-CSF plays an important role in regulating inflammatory responses and innate immune defense, particularly through activation of macrophages and neutrophils during infection or tissue injury. Rats are widely used as animal models for inflammatory and respiratory diseases, and GM-CSF–regulated myeloid responses have been studied in rat models of acute lung injury, where increased GM-CSF signaling influences macrophage activation and inflammatory cell recruitment in the lung, helping researchers better understand mechanisms of pulmonary inflammation and immune-mediated tissue damage relevant to human disease.