--- - |2 Abstract We have previously shown that treatment with recombinant human neuregulin‐1 (rhNRG‐1) improves pulmonary arterial hypertension (PAH) in a monocrotaline (MCT)‐induced animal model, by decreasing pulmonary arterial remodeling and endothelial dysfunction, as well as by restoring RV function. Additionally, rhNRG‐1 treatment showed direct myocardial anti‐remodelling effects in a model of pressure loading of the RV without PAH. This work aimed to study the intrinsic cardiac effects of rhNRG‐1 on experimental PAH and RV pressure overload, and more specifically on diastolic stiffness, at both the ventricular and cardiomyocyte level. We studied the effects of chronic rhNRG‐1 treatment on ventricular passive stiffness in RV and LV samples from MCT‐induced PAH animals and in the RV from animals with compensated and decompensated RV hypertrophy, through a mild and severe pulmonary artery banding (PAB). We also measured passive tension in isolated cardiomyocytes and quantified the expression of myocardial remodelling‐associated genes and calcium handling proteins. Chronic rhNRG‐1 treatment decreased passive tension development in RV and LV isolated from animals with MCT‐induced PAH. This decrease was associated with increased phospholamban phosphorylation, and with attenuation of the expression of cardiac maladaptive remodelling markers. Finally, we showed that rhNRG‐1 therapy decreased RV remodeling and cardiomyocyte passive tension development in PAB‐induced RV hypertrophy animals, without compromising cardiac function, pointing to cardiac specific effects in both hypertrophy stages. In conclusion, we demonstrated that rhNRG‐1 treatment decreased RV intrinsic diastolic stiffness, through the improvement of calcium handling and cardiac remodelling signalling. This article is protected by copyright. All rights reserved. - 'Clinical and Experimental Pharmacology and Physiology, Volume 0, Issue ja, -Not available-. '