ObjectiveWe compared diurnal patterns of blood pressure in Wistar–Kyoto (WKY) rats, spontaneously hypertensive rats (SHR) and stroke-prone spontaneously hypertensive rats (SHRSP), and analyzed the relationship between the change in diurnal patterns of blood pressure and target-organ damage in SHRSP.Materials and methodsBlood pressure, heart rate and motor activity in the three groups of rats were continuously monitored by radiotelemetry, from 1100 h on the first measuring day to 1300 h on the third measuring day. The left ventricular weight and the ratio of β-myosin heavy chain to α-myosin heavy chain in the cardiac left ventricle, morphological changes in the glomerular basement membrane in the kidney, 24 h urinary protein excretion and brain weights were also measured in 10-, 12-or 17-week-old SHRSP.ResultsThe SHR circadian blood pressure rhythm exhibited a pattern which peaked during the rats' active (light-off or dark) phase, but the peak time was a little closer to the resting (light-on) phase compared with that for WKY rats. Although the circadian blood pressure rhythm for 10-week-old SHRSP was similar to that observed for SHR, the patterns in 12-and 17-week-old SHRSP were shifted further towards the resting phase. Heart and left ventricular weight increased with the progression of hypertension. The ratio of β-to α-myosin heavy chain in the left ventricle was higher in 12-and 17-week-old SHRSP than in 10-week-old SHRSP. Brain weight was increased significantly in 17-week-old SHRSP compared with 10-and 12-week-old SHRSP. Increased urinary protein excretion and morphological changes in the glomerular basement membrane in the kidney were observed in 12-and 17-week-old SHRSP.ConclusionsThese data suggest that SHRSP have an abnormal circadian blood pressure rhythm associated with hypertensive target-organ damage. This rat strain may therefore be a useful model in which to investigate the mechanisms responsible for the alteration in the circadian blood pressure rhythm, and to analyze the relationship between the abnormal circadian rhythm and target-organ damage.