Glucocorticoids (GCs) are the most widely used immunosuppressive agents, but animals treated with GCs may experience deleterious side effects which limit their use in many clinical conditions. In the present study, we examined whether methylprednisolone sodium succinate (MPSS), a glucocorticoid, modulates circulating leukocyte numbers, phagocytic capacity and oxidative burst activity (OBA) of canine peripheral blood phagocytes, and whether tumor necrosis factor-alpha (TNF-$alpha$) release is affected by MPSS injection. Neutrophilia and monocytosis were induced by the administration of a high dose of MPSS, which is the recommended protocol for canine patients with acute spinal cord injury. The injection of MPSS decreased the phagocytic capacity of canine PMNs but not PBMCs, and recovered 12 hours (hr) after the completion of MPSS dosing. The OBA of both PMNs and PBMCs was suppressed by MPSS, and restored 24 hr after the completion of dosing. The lipopolysaccharide-induced TNF-α release by PBMCs but not PMNs exposed to MPSS was reduced 12 hr after the completion of dosing, and recovered 48 hr after the completion of dosing. These results suggest that the application of MPSS protocol inhibits the innate immune functions of canine peripheral blood phagocytes for short time relatively.