Laser Doppler flow meters are used to provide continuous records of blood flow in tissues. An account is given of their principles of operation and of the differences between two of the commercially available instruments: the Periflux model PF3 and the Moor blood flow monitor model MBF3D. The Periflux tended to have higher noise levels and to drift more than the Moor. The principal disadvantage of these instruments is that they cannot be calibrated in absolute units of blood flow. It is suggested that the method of standardization adopted for the Periflux be used with other machines, based on the signal produced by the Brownian motion of a 0.5% suspension of 0.48 micron diameter polystyrene microspheres at 20 degrees C. The signal representing zero flow in a tissue can be obtained by recording with the same light intensity from a stationary reflector. In model systems the instruments are linear with red cell volume fractions up to only 1%, which is below that likely to be encountered in many tissues. The effects of ambient light and the origin of movement artifacts are also considered.

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