The uniformity of regional liver perfusion was studied in eight anaesthetised dogs by measuring the clearance of 85Kr with three beta detectors, and the results were compared with independent measurements of mean perfusion by electromagnetic flowmetry. The intra-hepatic flow distribution was found to be heterogeneous, with a coefficient of variation between locations which averaged 17%. The pattern of flow varied from liver to liver, and there was no consistent tendency for any one of a limited number of locations to be better or less well perfused than any other. Discrepancies between 85Kr clearance and flowmeter measurements were of a similar magnitude to inter-regional variations in flow, and there was no evidence of a systematic difference between the two measurement techniques. Individual 85Kr clearance measurements can misrepresent the perfusion of the liver as a whole, and averaging several measurements performed in different locations is likely to yield a better estimate of mean flow.