Actual or simulated microgravity causes a cephalad fluid shift toward the upper body [1,2] that alters hemodynamics in the head and induces characteristic symptoms such as facial edema and nasal congestion. These symptoms occur in astronauts during spaceflight [1], and in subjects exposed to head-down tilt (HDT) [3,4], a method commonly used to simulate microgravity on Earth.The cephalad fluid shift also changes hemodynamics in the brain. Several studies using transcranial Doppler (TCD) ultrasound have shown that the velocity of cerebral blood flow increases over several hours after the onset of parabolic flight [5] or HDT [6]. A postural change from an upright to a supine position also increases tissue oxygenation in the brain [7]. These observations suggest that exposure to HDT increases cerebral blood flow (CBF), at least in the early phase. On the other hand, an image analysis using single photon emission computer tomography (SPECT) demonstrated that a significant increase in CBF occurred in the basal ganglia and the cerebellum, but not in the cerebral hemispheres [8]. In that study, however, CBF was measured only once within 5 min from the onset of HDT. The SPECT experiments have the advantage of simultaneously measuring CBF in several brain regions. However, the disadvantage of SPECT is that it cannot measure CBF continuously. It is important to know the time course of the changes in CBF, because an autoregulatory mechanism may constantly modify the effect of HDT on CBF. Thus the first purpose of the present study was to measure CBF continuously with a laser Doppler flowmeter in rabbits exposed to HDT. We hypothesized that CBF would increase immediately after the onset of HDT and decrease toward the pre-HDT baseline level during 1 h of HDT. Japanese Journal of Physiology, 52, 105-110, 2002 Key words: cerebral blood flow, head-down tilt, microgravity, somatosensory-evoked potentials, laser Doppler flowmetry.Abstract: Changes in cerebral blood flow (CBF) and somatosensory-evoked potentials (SEPs) were studied in rabbits exposed to headdown tilt (HDT) at 45°and 75°. The animals were anesthetized with alpha chloralose and the lungs were artificially ventilated. CBF was continuously measured by laser Doppler flowmetry (LDF), and SEPs were recorded as responses of the cortex to median nerve stimulation. In the 45°HDT rabbits, CBF did not change significantly in the parietal cortex during 1 h of HDT. In contrast, in the 75°HDT rabbits, CBF did not change significantly within 5 min after the onset of HDT, but decreased gradually to 79% of the pre-HDT baseline value at the end of 1 h of HDT. The latency and amplitude of SEPs did not change significantly throughout the experiment in any group. These results suggest that CBF and SEPs do not change significantly during 1 h of 45°HDT and that 75°HDT disturbs the regulation of the cerebral circulation but does not affect cortical somatosensory response, at least for 1 h.