The present study was designed to investigate the role of cytokines in the pathogenesis of Babesia caballi in experimentally infected horses. The expression of cytokine mRNA was determined by using reverse transcription-polymerase chain reaction in two B. caballi-infected horses for 2 weeks after the infection. In one horse, there was up-regulation of interferon-gamma, tumor necrosis factor-alpha (TNF-alpha) and interleukin-2 mRNAs, while in the second horse, expression of only TNF-alpha mRNA was up-regulated. No change was observed in interleukin-4 mRNA in both of the horses. To know the relation between nitric oxide (NO) production and pathogenesis, NO production was assayed in three dexamethasone treated-B. caballi-infected horses. Production of NO in all 3 horses increased significantly before death, although the parasitemia level remained very low. Treatment with NO inhibitor resulted in the suppression of NO production and increased parasitemia level in a horse, which died of the infection. The pathological examination showed that the main cause of the death was dyspnoea and pulmonary edema. Histopathologically, diffuse global mesangial proliferative glomerulonephritis was also observed. These results suggested that NO may be a critical effector molecule of immune defense against parasite. TNF-alpha and NO might be contributing to the pathogenesis in B. caballi infection.