To examine whether spontaneous autoregulatory responses of the arterial vasculature can contribute to the short-term variability of arterial pressure (AP), rats were chronically instrumented for the simultaneous measurement in the conscious State of AP and of either cardiac output, mesenteric or hindquarters blood flow. Acute pharmacological removal of neurohumoral influences induced a marked increase in AP variability when the AP level was maintained with a continuous noradrenaline infusion. AP lability was especially due to the spontaneous occurrence of large depressor episodes that were accompanied by systemic and regional vasodilations. Both time- and frequency-domain analyses revealed that changes in vascular conductances lagged by ∼1 s behind AP changes, thus demonstrating the autoregulatory-like nature of these hemodynamic fluctuations. In intact animals, changes in vascular conductances were not delayed relative to AP changes, except in the mesenteric circulation, where autoregulatory-like fluctuations prevailed up to 0.1 Hz. In this circulation, most of the spontaneous variability of vascular conductance appeared as an oscillation centered at 0.1-0.15 Hz, irrespective of neurohumoral influences. We conclude that after neurohumoral blockade, autoregulatory-like responses become a major source of AP variability. Due to their unexpectedly short latency, these responses operate up to 0.15-0.2 Hz. The reflex control of AP normally overrides autoregulation, except in the mesenteric circulation.