Abstract-There are several controversies concerning the enhanced gene expression of cardiac renin-angiotensin system components in spontaneously hypertensive rats (SHR) compared with their normotensive control Wistar-Kyoto (WKY) rats. We hypothesized that these discrepancies arise from circadian fluctuations in gene expression. We examined the circadian mRNA expression of renin, angiotensinogen, ACE, and angiotensin type 1a (AT1a) and type 2 (AT2) receptors in the hearts of SHR and WKY rats by real-time quantitative reverse transcription-polymerase chain reaction. The cardiac mRNA expression of the renin-angiotensin system components showed circadian oscillations in both SHR and WKY rats. The amplitudes of these circadian fluctuations were greater in the SHR than in the WKY rats. The mRNA levels of the renin-angiotensin system components were also increased in the SHR compared with the WKY rats at many time points (especially during the dark phase). However, the levels of ACE, AT1a receptor, and AT2 receptor mRNA in the SHR and WKY rats were almost the same during the late light phase. In contrast to mRNA expression, ACE activity was similar both at the time of maximum and minimum mRNA expression. The AT1 receptor antagonist candesartan upregulated AT1a receptor mRNA and downregulated ACE mRNA at specific time points only in the SHR group. Our findings of differential diurnal expression of cardiac renin-angiotensin system genes in SHR and WKY rats appear to explain the discrepancies between prior studies. However, the physiological relevance of the differential circadian mRNA expression of the renin-angiotensin system components remains to be elucidated. Key Words: circadian rhythm Ⅲ renin-angiotensin system Ⅲ renin Ⅲ angiotensinogen Ⅲ angiotensin-converting enzyme Ⅲ receptors, angiotensin II Ⅲ rats, spontaneously hypertensive M any studies in the last 3 decades have found that the cardiac renin-angiotensin system (RAS) is independently regulated from the systemic RAS and plays important pathophysiologic roles in cardiac hypertrophy and remodeling. 1-3 Many studies have examined the differences in the cardiac RAS in spontaneously hypertensive rats (SHR) and their normotensive counterpart, the Wistar-Kyoto (WKY) rat; these studies have shown some discrepancies in the expression of RAS component genes in SHR and WKY rats 4 -8 (Table 1). Some data have indicated an increased mRNA expression of angiotensinogen, ACE, and angiotensin type 1a (AT1a) receptors in SHR compared with WKY rats, whereas other studies have found no differences between the two strains.Many physiological processes are under the control of a biologic clock. In mammals, the central circadian pacemaker is located in the suprachiasmatic nucleus of the brain, whose phase is directly light-entrained by the retina. 9 Recent experiments have shown that peripheral tissues also have oscillatory systems that contain a transcriptional-translational feedback loop of clock genes. 9 -11 The heart also contains fully functional internal clocks, and several car...