Fatigue is a persistent and debilitating symptom following radiation therapy for prostate cancer. However, it is not well-understood how radiation targeted to a small region of the body can lead to broad changes in behavior. In this study, we used targeted pelvic irradiation of healthy male mice to test whether inflammatory signaling mediates changes in voluntary physical activity levels. First, we tested the relationship between radiation dose, blood cell counts, and fatigue-like behavior measured as voluntary wheel-running activity. Next, we used oral minocycline treatments to reduce inflammation and found that minocycline reduces, but does not eliminate, the fatigue-like behavioral changes induced by radiation. We also used a strain of mice lacking the MyD88 adaptor protein and found that these mice also showed less fatigue-like behavior than the wild-type controls. Finally, using serum and brain tissue samples, we determined changes in inflammatory signaling induced by irradiation in wild-type, minocycline treated, and MyD88 knockout mice. We found that irradiation increased serum levels of IL-6, a change that was partially reversed in mice treated with minocycline or lacking MyD88. Overall, our results suggest that inflammation plays a causal role in radiation-induced fatigue and that IL-6 may be an important mediator. Keywords: CCL, chemokine (CC) ligand; CD30 L, CD30 ligand; CFS, chronic fatigue syndrome; CRF, cancer-related fatigue; CXCL, chemokine (CXC) ligand; Cancer-related fatigue; Cytokines; FGF, fibroblast growth factor; Fas-L, Fas Ligand; Fatigue; G-CSF, granulocyte colony-stimulating factor; GM-CSF, granulocyte-macrophage colony-stimulating factor; ICAM, intercellular adhesion molecule; IFN, interferon; IL, interleukin; Inflammation; LIF, leukemia inhibitory factor; M-CSF, macrophage colony-stimulating factor; MCV, mean corpuscular volume; Minocycline; MyD88, myeloid differentiation primary response 88 protein; PDGF-bb, platelet-derived growth factor subunit B; RANTES, regulated on activation normal T cell expressed and secreted; RBC, red blood cell; Radiotherapy; TIMP, tissue inhibitor of metalloproteinases; TLR, toll-like receptor; TNF, tumor necrosis factor; VEGF, vascular endothelial growth factor; VWRA, voluntary wheel running activity; Voluntary wheel-running activity; WBC, white blood cell; myd88.