Abstract: The percentage of sporozoites developed in exoerythrocytic forms in the rats treated intravenously with cyclophosphamide 24 hrs before inoculation of sporozoites was higher than that in untreated controls, so was the mean size of exoerythrocytic form. Cyclophosp-hamide could decrease the sensitivity of hepatocytes to the vector's tissues inoculated together with sporozoites, thus decreasing the incidence of cloudy swelling of hepatocytes induced by the vector's tissues and inhibiting the white blood cells infiltration induced by the ruptured mature exoerythrocytic form.The fact that cyclophosphamide might enhance the invasion of sporozoites into the hepatocytes indicates that the phagocytic activity of Kupffer's cells to destroy the sporozoites might be inhibited by cyclophosphamide.