Peptidyl‑prolyl cis/trans isomerase, NIMA-interacting 1 (Pin1) is a member of a large superfamily of phosphorylation‑dependent peptidyl‑prolyl cis/trans isomerases, which not only regulates multiple targets at various stages of cellular processes, but is also involved in the pathogenesis of several diseases, including microbial infection, cancer, asthma and Alzheimer's disease. However, the role of Pin1 in cardiac fibrosis remains to be fully elucidated. The present study investigated the potential mechanism of Pin1 in isoprenaline (ISO)‑induced myocardial fibrosis in rats. The rats were randomly divided into three groups. Echocardiography was used to evaluate changes in the size, shape and function of the heart, and histological staining was performed to visualize inflammatory cell infiltration and fibrosis. Reverse transcription‑quantitative polymerase chain reaction analysis, immunohistochemistry and Picrosirius red staining were used to differentiate collagen subtypes. Additionally, cardiac‑specific phosphorylation of mitogen‑activated protein kinase kinase 1/2 (MEK1/2) and extracellular‑signal regulated protein kinase 1/2 (ERK1/2), and the activities of Pin1 and α‑smooth muscle actin (α‑SMA) and other oxidative stress parameters were estimated in the heart. The administration of ISO resulted in an increase in cardiac parameters and elevated the heart‑to‑body weight ratio. Histopathological examination of heart tissues revealed interstitial inflammatory cellular infiltrate and disorganized collagen fiber deposition. In addition, lipid peroxidation products and oxidative stress marker activity in plasma and tissues were significantly increased in the ISO‑treated rats. Western blot analysis showed significantly elevated protein levels of phosphorylated Pin1, MEK1/2, ERK1/2 and α‑SMA in remodeling hearts. Treatment with juglone following intraperitoneal injection of ISO significantly prevented inflammatory cell infiltration, improved cardiac function, and suppressed oxidative stresses and fibrotic alterations. In conclusion, the results of the present study suggested that the activation of Pin1 promoted cardiac extracellular matrix deposition and oxidative stress damage by regulating the phosphorylation of the MEK1/2‑ERK1/2 signaling pathway and the expression of α‑SMA. By contrast, the inhibition of Pin1 alleviated cardiac damage and fibrosis in the experimental models, suggesting that Pin1 contributed to the development of cardiac remodeling in ISO‑administered rats, and that the inactivation of Pin1 may be a novel therapeutic candidate for the treatment of cardiovascular disease and heart failure.