The present study is based on the hypothesis that the development of essential hypertension is due to a defect in cardiovascular control. Young, prehypertensive, spontaneously hypertensive (SHR) rats were compared with normotensive Wistar Kyoto (WKY) rats . Spectral analysis of blood pressure (ABP) fluctuations previously demonstrated that baseline low-frequency fluctuations (0.04-0.1 Hz) are reduced in the SHR rats even before they develop hypertension. In the present study, 20-min-long ABP traces were sampled with the focus on frequencies as low as 0.004 Hz. The effect caused by a perturbation in blood pressure was investigated in young SHR versus WKY rats. A sudden drop in blood volume (2-ml bleeding) caused a clear increase in low-frequency ABP fluctuations, more pronounced in SHR than in WKY rats. The decrease in mean ABP was larger in SHR (-42%) than in WKY rats (-21%). Alpha1-blockade (prazosin) affected the reaction of both species to bleeding differently. The results indicate that the SHR rats exhibit an abnormal nervous response (α-sympathetic) to a drop in blood pressure, which might be at the basis of the development of hypertension.