The anti-inflammatory effect of methotrexate (MTX) is mediated by the increasing extracellular concentration of adenosine. The response of the immune system to activation of cell membrane subclass receptors for adenosine initiates intracellular signaling pathways, which causes immunomodulatory responses. To test the direct immunomodulatory effects of systemic administration of adenosine on animal model of inflammation, zymosan induced arthritis (ZIA) in mice was employed. Sterile zymosan (30 mg) was injected directly into the knee cleft of 60 mice, divided into 3 equal groups. The experimental groups received, every second day, 0, 0.25, and 0.5 mg/kg adenosine intraperitoneally. Inflammatory intensity was evaluated clinically by knee swelling measurement, histology, the total white blood cell (WBC) count, and serum tumor necrosis factor-alpha (TNF-a) levels served as markers of systemic inflammatory reaction. As observed, only the mice that received the higher dose of 0.5 mg/kg adenosine developed a significantly milder arthritis. A 23.9% reduction (P o 0.004) of the mean knee diameter swelling was noted. The histological inflammatory parameters and WBC count in the 0.5 mg/kg adenosine-treated mice were also decreased (6,555 7 510/mm 3 vs. 9,250 7 530 WBC/mm 3 in the untreated mice, P 4 0.006; 9,188 7 588 WBC/mm 3 in the 0.25 mg/kg adenosine-treated group). Serum TNF-a levels were significantly reduced (78.1 pg/ml in the 0.5 mg/kg adenosine-treated group vs. 124.3 pg/ml, in the control group, P o 0.004). The data indicate a remarkable clinical beneficial effect, as well as a local and systemic anti-inflammatory response that was noted on administration of 0.5 mg/kg adenosine every alternate day to mice with inflammatory arthritis. Drug Dev.