Leflunomide, a novel immunosuppressant, has been the subject of recent preclinical studies using solid organ allo- and xenotransplantation models. The objectives of this study were to evaluate the efficacy and toxicity of leflunomide using a rat cardiac allotransplant model in two different strain combinations (DA x PVG and DA x Lew). Leflunomide, at doses ranging between 5 and 30 mg/kg, prolonged graft survival in both strain combinations as effectively as CsA and FK506 1 mg/kg (P < 0.05). A dose-dependent effect was seen only after a longer treatment course. When ongoing rejection was intercepted early (postoperative day 2), 5 mg/kg was as effective as 1 mg/kg FK506 (P > 0.05) but was inferior to CsA in the DA x PVG combination (P < 0.05). However, in the DA x Lew combination, leflunomide was equally as efficacious as 15 mg/kg CsA and 1 mg/kg FK506 (P > 0.05). If ongoing rejection was treated at postoperative day 4, 10 mg/kg leflunomide was not only as effective as 15 mg/kg CsA and 1 mg/kg FK506, but demonstrated a dose-dependent increase in graft survival in both strain combinations. The toxicity of leflunomide at doses of especially 5-20 mg/kg was minimal in comparison to therapeutic doses of CsA and FK506 using body weight and biochemical parameters of renal and liver function. These in vivo observations convincingly show leflunomide to be equally as potent an immunosuppressant as CsA and FK506 in transplant rejection. It is also well tolerated on long-term administration and, by virtue of this fact, is a potentially suitable candidate for clinical transplantation.