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To study the spontaneous variability in regional haemodynamics.Twenty normotensive Wistar-Kyoto rats were chronically instrumented with an arterial catheter and with pulsed Doppler flowmeters on the distal aorta, and the superior mesenteric and left renal arteries. After surgical recovery, the rats were monitored in unrestrained conditions. The recorded signals were analysed beat-to-beat to obtain means and coefficients of variation for mean arterial pressure, heart rate, regional blood flow velocity (consecutive 0.8-s periods) and indices of regional vascular resistance (0.8-s ratio of mean arterial pressure to mean blood flow velocity).Muscle and splanchnic blood flow velocities were markedly variable, with coefficients of variation of 12.8 ±0.8 and 12.2 ±1.7% (means ±SEM), respectively, about twice as large as the coefficient of variation for mean arterial pressure (6.2 ±0.3%). The renal blood flow velocity was slightly less variable than the muscle and splanchnic blood flow velocities, with a coefficient of variation of 10.4 ±0.8%, but still markedly and significantly more variable than systemic arterial pressure. A contingency analysis of paired variations in any two given parameters (arterial blood pressure, heart rate, blood flow velocities and indices of vascular resistance) showed a concordant pattern, the only exception being a distinctly discordant trend for the covariations in muscle and splanchnic blood flow velocities. Conclusions: Regional blood flow velocity and vascular resistance have a larger degree of spontaneous variability than systemic arterial pressure. Renal blood flow velocity is also highly variable, suggesting that short-term stimuli that affect the renal blood vessels are not countered by autoregulation to any great degree. We conclude that while central factors may drive concordant regional haemodynamic variations, some opposing changes in regional blood flow velocity may cancel each other out, thereby reducing the variability in systemic blood pressure.