The control of arterial blood pressure (ABP) by the major pressor mechanisms in hypertension is investigated. One-month old, spontaneously hypertensive rats (SHR) and their normotensive controls (WKY) were investigated. At this young age, SHR are still normotensive. The response to increasing doses of converting enzyme inhibitor (CEI, captopril), followed by acute hemorrhage, is analyzed. Spectral analysis of the slow ABP fluctuations was performed in a frequency range of 0.004-0.2 Hz. Administration of CEI did not alter mean ABP nor ABP fluctuations in both strains of rats. Under baseline conditions as well as under increasing doses of CEI, SHR, as compared to their WKY controls, displayed reduced low frequency fluctuations. Hemorrhage under CEI induced in SHR a moderate increase in power at the 0.004-0.07 Hz range as compared to the marked increase when bled at baseline. This may suggest that the renin-angiotensin system (R-A) contributes significantly to the exaggerated response to bleeding. A frequency specificity in the power spectrum is observed. α control is reflected in the 0.04-0.07 Hz frequency range, whereas the R-A system contributes to ABP fluctuations in a wider frequency range, 0.004-0.07 Hz.