New Findings What is the central question of this study? Does oxidative stress induce impairment of autophagy that results in myocyte hypertrophy early after pressure overload? What is the main finding and its importance? In cultured myocytes, hydrogen peroxide decreased autophagy and increased hypertrophy, and inhibition of autophagy enhanced myocyte hypertrophy. In rats with early myocardial hypertrophy after pressure overload, myocyte autophagy was progressively decreased. The antioxidant N‐acetyl‐cysteine or the superoxide dismutase mimic tempol prevented the decrease of myocyte autophagy and attenuated myocyte hypertrophy early after pressure overload. These findings suggest that oxidative stress impairs myocyte autophagy that results in myocyte hypertrophy. AbstractInsufficient or excessive myocyte autophagy is associated with left ventricular (LV) hypertrophy. Reactive oxygen species mediate myocyte hypertrophy in vitro and pressure overload‐induced LV hypertrophy in vivo. In the present study, we tested the hypothesis that oxidative stress induces an impairment of autophagy that results in myocyte hypertrophy. H9C2 cardiomyocytes pretreated with the autophagy inhibitor 3‐methyladenine were exposed to 10 and 50 μm hydrogen peroxide (H2O2) for 48 h. Male Sprague–Dawley rats underwent abdominal aortic constriction (AAC) or sham operation. The animals were killed 24, 48 or 72 h after surgery. In a separate group, the AAC and sham‐operated rats randomly received the antioxidant N‐acetyl‐cysteine or the superoxide dismutase mimic tempol for 72 h. In H9C2 cardiomyocytes, H2O2 decreased the ratio of microtubule‐associated protein light chain 3 (LC3) II to LC3 I and increased P62 and phosphorylated ERK (p‐ERK) proteins and myocyte surface area. 3‐Methyladenine further increased H2O2‐induced p‐ERK expression. In rats after AAC, the heart to body weight ratio was progressively increased, the LC3 II/I ratio was progressively decreased, p62 and p‐ERK expression was increased, and expression of Beclin1, Atg5 and Atg12 was decreased. N‐Acetyl‐cysteine or tempol prevented the decreases in the LC3 II/I ratio and Beclin1 and Atg5 expression and attenuated the increases in LV wall thickness, myocyte diameter and brain natriuretic peptide expression in AAC rats. In conclusion, oxidative stress decreases Beclin1 and Atg5 expression that results in impairment of autophagy, leading to myocyte hypertrophy. These findings suggest that antioxidants or restoration of autophagy might be of value in the prevention of early myocardial hypertrophy after pressure overload.