Left ventricular hypertrophy (LVH) is an adaptive change in response to hypertensive pressure overload. Some evidence indicates that the decrease in sarcoplasmic reticulum (SR) Ca²⁺-ATPase mRNA expression, which may contribute to a diastolic dysfunction of the heart, occurs in the experimental pressure overload model. Also, recent studies have demonstrated that angiotensin II (Ang II) and angiotensin II receptor type 1 (AT₁) play important roles in LVH. The purpose of this study was to investigate the function of the SR and the role of AT₁ in genetic hypertension in spontaneously hypertensive rats (SHR) at ages 10 and 18 weeks. In SHR, cardiac hypertrophy has already developed at 10 weeks of age. SR Ca²⁺-ATPase activity and mRNA expression were significantly lower in SHR than in Wistar-Kyoto rats (WKY). Plasma renin activity in SHR was unchanged compared with WKY, whereas the Ang II concentration in SHR was significantly higher than that in WKY. AT₁ mRNA expression in SHR was similar to that in WKY. These results suggest that in the early stage of hypertension in SHR Ang II may stimulate hypertrophy in the cardiomyocytes through the AT₁, which is not downregulated by a high concentration of Ang II. (Jpn Circ J 1999; 63: 367 - 372)