A comparison has been made of the plasma clearances of the anionic dye analogues, tetrabromsulphthalein (BSP) and dibromsulphthalein (dBSP), analysed by means of a two-compartmental model. In the normal conscious dog all three transport rate constants and initial and steady state plasma clearances were greater with dBSP than BSP. On the other hand the equivalent liver volume (ELV) and storage capacity were lower with dBSP. In patients with various hepatic disorders dBSP values were greater than those obtained with BSP apart from rate constant b (liver to plasma). Patients with advanced liver impairment in general exhibited lower hepatic uptake, clearance and storage of both dyes compared to patients with mild forms of liver disease. The ratio of ELV values for BSP and dBSP appeared to be related to overall hepatic function, with ratios greater than 1 in the normal dogs and less than 1 in the patients.