Abstract: To examine the pathogenesis of pregnancy-induced hypertension syndrome（PIH） in rat and the feasibility of treating it with L-arginine. The rat PIH model induced by methyl ester was established, and treated with L-arginine. The therapeutic efficacy on rat PIH and the changes of the indexes such as NO, NOS, ET, ANG II，TXA₂ and PGI₂ were detected. The symptoms and physical signs of rat PIH were obviously ameliorated by administration of L-arginine. Compared with control group，low dosage group and high dosage group，the content of serum NO，ANG II and PGI₂，the activity of rat NOS in model group were decreased, while ET，and TXA₂ increased.So，the pathogenesis of PIH are the decrease of level of NO, activity of NOS and dise-quilibria of blood vessel regulating factors. The fact that the L-arginine is effective for PIH may provides a new reasonable and feasible way for PIH therapy.