Hepatic blood flow was measured in the unanesthetized rabbit using the clearance technique of 198Au and 125I RB. The values are: 71.82 +/- 16.24 ml-min-1-kg-1 for 198Au, and 60.21+/-9.94 ml-min-1-kg-1 for 125I RB (P less than 0.01). The overestimation of HBF with colloidal gold is probably due to extra splanchnic sequestration which has been estimated to be 15+/-3%. The week extraction of RB limits the use of this dye for measurements of HBF. HBF is easier to measure with 198 Au.