AIM: To investigate whether the circadian mRNA expression of the renin-angiotensin system (RAS) components (ATNG、AT_1R、AT_2R) in myocardium is correlated with left ventricular hypertrophy in both spontaneously hypertensive rats (SHR) and renal hypertensive rats (RHR). METHODS: The model of RHR was established using 2K1C (two-kidney one clip). SHR, RHR, and normotensive control rats (CON) were housed in a separate environment-controlled room for two weeks, in which a strict 12-hour light/12-hour dark cycle regimen was enforced (lights on at 900, zeitgeber time [ZT0]). Rats were killed every 4 hours from 900 am (ZT0) to 500 pm (ZT20) on three separate days. We examined the circadian mRNA expression of ATNG、AT_1R、AT_2R in the hearts of SHR, RHR and CON rats by RT-PCR. RESULTS: Heart weight to body weight ratios and left ventricle weight to heart weight ratios indicated a significant increase in SHR and RHR compared with normotensive rats. The cardiac mRNA expression of the RAS components showed circadian oscillations in SHR, RHR and CON rats. The amplitudes of these circadian fluctuations were greater in the SHR and RHR than in the CON rats. The mRNA levels of the RAS components were increased in the SHR and RHR compared with the CON rats at many time points. CONCLUSION: The data indicate that overexpression and enhanced amplitudes of circadian fluctuations of RAS components in SHR and RHR may play a role in the development of left ventricular hypertrophy.