FTY720 is a new immunosuppressant agent and selectively decreases the number of circulating lymphocytes. In this study, we compared the effects of FTY720 with those of tacrolimus on experimental autoimmune myocarditis (EAM) in rats. A significant decrease in circulating lymphocyte counts was noted after a single administration of FTY720 in normal rats. At day 0, 7-week-old male Lewis rats were immunized with purified porcine cardiac myosin emulsified in complete Freund's adjuvant. FTY720 or tacrolimus was administered intraperitoneally daily. The number of myocarditis-affected areas in the FTY720 treatment groups with doses of 0.1 mg/kg/day was significantly lower than those in control groups at days 14 and 28. In addition, at day 28, the myocarditis-affected areas in the FTY720 treatment group were significantly smaller than those in the tacrolimus treatment group receiving the same dose. Effects of early administration (days 0-10) and delayed administration (days 11-20) of FTY720 also were examined. At day 28, the myocarditis-affected areas in the early therapy group were significantly lower than those in the control group. In conclusion, we demonstrated that the development of EAM could be prevented by FTY720. These data also indicated that lymphocyte-mediated immunity is critically involved in the development of EAM.