Continuous measurement of CBV and CBF by means of laser-Doppler flowmetry was performed to analyse the cerebrovascular response to water accumulation during brain infusion oedema in cats. The presence of an oedema generator elicits a continuous reduction of CBV and a transient reduction of CBF. With cessation of this mechanical force of oedema production, CBV and CBF returned to control levels. These results indicate that excess accumulation of water in brain tissue by itself does not decrease CBF even when the driving pressure of oedema fluid is present, but that brain oedema attenuates CBV only when a pressure gradient exists. These results suggest that there are microcirculatory changes in brain oedema before and after removal of the oedema generating focus. The laser-Doppler flow method is a useful technique for the continuous assessment of tissue perfusion.