Rats were fed on distilled water (DW) or DW containing beraprost sodium (BPS). BPS concentration was 1.5 or 3.0 ,u /ml in DW for the low BPS groups and 6.0 or 10.0 p /ml in DW for the high BPS group. Monocrotaline (MCT) was subcutaneously given (60 mg/kg), and saline was injected as control. Data were analyzed among the following groups; Groups (Saline+DW), (Saline+Low BPS), (MCT+DW), (MCT+Low BPS) and (MCT+High BPS). Three weeks later, pulmonary (Ppa) and systemic (Psa) arterial pressure were measured under anesthesia. MCT caused significant elevation of Ppa { 18.3 ±0.6 cmH2 0 for Group ( Saline + DW) vs. 27.2 ± 1.2 cmH2O for Group (MCT+DW), p<0.001, mean±s.E.} and Ppa was significantly and dose-dependently suppressed by BPS; Group (MCT+DW) vs. Groups (MCT+Low BPS), 23.4±0.7 cmH2O and (MCT+High BPS), 22.5+0.5 cmH2O, p <0.05, mean±s.E.). Psa was not lowered dose-dependently by BPS. We conclude that oral beraprost sodium suppresses pulmonary hypertension produced by monocrotaline in rat.monocrotaline, beraprost sodium, pulmonary hypertension A single subcutaneous injection of the monocrotaline (MCT) results in pulmonary hypertension (Kay et al. 1967). It has been known that platelets are responsible for the development of pulmonary arterial injury in this model. MCT induces pulmonary sequestration of platelets and thrombocytopenia (Hilliker et al. 1982(Hilliker et al. , 1984. A single intravenous injection of prostacyclin (PGI2) at the time of MCT administration inhibits MCT-induced immediate pulmonary hypertension (Czer et al. 1986). To test the hypothesis that a long-term administration of a PGI2 derivative, beraprost sodium (BPS), suppresses MCT-induced pulmonary hypertension, we measured pulmonary arterial pressure (Ppa) three weeks after MCT-treatment in rats fed on distilled water (DW) with or without BPS.