The single fibre laser Doppler flowmetry technique has been designed to perform continuous quantification of microvascular perfusion deep in tissue. In this study we have evaluated the use of the technique in renal tissue. A total number of 164 recordings were analyzed. Fluctuations in perfusion related to heart rate and respiration were observed. The possible nature of the low-frequency flow motion waves (10-12 min-1) is discussed. No significant difference in perfusion level could be detected between the upper, the middle and the lower parts of the kidneys (p greater than 0.69), or between the cortex and the medulla (p = 0.77). The spatial variation, expressed as the difference between two consecutive measurements, was large. The median and mean values of groups of data are however reproducible. When the renal artery is reopened after 30 sec of occlusion, the pre-occlusive flux levels are regained after approximately 1.5 sec. The single fibre laser Doppler flowmetry technique can be used for renal microcirculatory studies, but comparative studies with other methods have to be performed.