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Rhythmic variations in blood flow have been observed in various vascular beds, including brain. We have characterized fluctuations of the microcirculation in different locations in the brain, and studied the response to changes in arterial carbon dioxide tension, arterial pressure, and cerebrospinal fluid pressure. Laser Doppler flowmetry was performed in 20 pentobarbital-anesthetized pigs. Flow probes were positioned on the brain surface and 3, 10, and 20 mm into the cerebral tissue. The protocol included carbon dioxide breath- ing, hemorrhagic hypotension, and infusion into the cisterna magna. Twenty-five periods of low-frequency oscillations (4.5/min) were found in 10 pigs with superimposed spindle-shaped rhythmic variations (0.5/min) of the amplitude in 7. There were no rhythmic changes in arterial pressure or intracranial pressure. Rhythmic activity was most often seen in the probe positioned 20 mm into the brain and was often seen in several probes at the same time. Animals with rhythmic oscillations before interventions had lower cerebral perfusion pressure and arterial pressure, lower heart rate, and higher laser Doppler signal than the others. Blood loss often initiatied oscillations. High intracranial pressure tended to abolish preexisting oscillations. Hypercapnia always abolished preexisting oscillations. Oscillations were more frequent if the cerebral perfusion pressure was in the low range of cerebral autoregulation, occurred more often in the cerebral locations with relatively high local flow, were most likely to be localized, and therefore probably caused by local metabolic or myogenic variations.