Abstract Body (Do not enter title and authors here): Background: Pulmonary arterial hypertension (PAH) is a progressive disease characterized by pulmonary arterial remodelling, leading to right ventricular failure. Accumulating evidence shows the importance of inflammation in the pathogenesis of PAH. Regnase-1 is a ribonuclease essential in limiting inflammation by degrading messenger RNAs involved in immune responses, such as interleukin-6 (IL-6) and IL-1β. This occurs through recognition of stem-loop structures in the 3′ untranslated regions of target mRNAs by Regnase-1. We previously reported that Regnase-1 mRNA expression was reduced in peripheral blood mononuclear cells of PAH patients and that its expression was inversely correlated with the severity of the disease. In addition, myeloid-specific deletion of Regnase-1 in mice led to the spontaneous development of severe PAH, accompanied by enhanced accumulation of inflammatory cells in the lungs (Circulation. 2022, 146(13):1006). We hypothesized that inducing increased expression of Regnase-1 could ameliorate the pathology of pulmonary hypertension (PH). Methods: We designed antisense phosphoramidate morpholino oligonucleotides (MOs) that bind to the stem-loop structure within the 3′ UTR of Regnase-1 mRNA. We previously demonstrated that the designed MOs suppress Regnase-1 self-degradation in mice and human cells (Sci Transl Med. 2022,14(644):eabo2137). We developed additional MOs that specifically bind to Regnase-1 mRNA in rats, inhibiting self-degradation by Regnase-1. We then examined the effects of these Regnase-1-targeted MOs on PH pathology in both a hypoxia-induced PH mouse model and a monocrotaline-induced PH rat model. Results: Intratracheal administration of Regnase-1-targeting MOs improved PH pathology in the hypoxia-induced PH mouse and monocrotaline-induced PH rat models. We confirmed that Regnase-1-targeting MO administration suppressed the decline in Regnase-1 mRNA expression in the lungs of each PH model. Regnase-1-targeting MO administration significantly reduced the increase in inflammatory cytokine expression, particularly IL-6 mRNA expression, in the lungs. Conclusions: Intratracheal administration of Regnase-1-targeting MOs increased Regnase-1 mRNA expression and suppressed inflammation in the lungs of PH model animals, thereby improving PH pathology. Disrupting the Regnase-1 self-regulation pathway with MOs might become a potential therapeutic strategy to enhance Regnase-1 abundance and suppress inflammation in PH patients.