The uncoupling activity of a narrow-spectrum benzimidazole anthelmintic triclabendazole (TCZ, 6-chloro-5-[2,3-dichlorophenoxy]-2-methylthio-benzimidazole) and its 2 principal metabolites triclabendazole sulfoxide (TCZ sulfoxide, 6-chloro-5-[2,3-dichlorophenoxy]-2-methylsulfinyl-benzimidazole) and triclabendazole sulfone (TCZ sulfone, 6-chloro-5-[2,3-dichlorophenoxyl]-2methylsulfonyl- benzimidazole) has been determined using rat liver mitochondria. With glutamate or succinate as the mitchondrial substrate, and the respiratory control index (RCI) as an indicator of uncoupling activity, we found that TCZ and its 2 main metabolites were uncouplers of oxidative phosphorylation at micromolar concentrations. The rank order of in vitro activity was TCZ sulfone > TCZ sulfoxide > TCZ. Structure-activity relationship studies revealed that the electron-withdrawing power of the substituent in the 2-position was the principal determinant of mitochondrial uncoupling activity. Correlation techniques were used to assess the strength of the relationship between the ability of TCZ and its metabolites to uncouple oxidative phosphorylation and their ability to lower the electrical resistance of planar bimolecular lipid membranes. A log-log plot of RCI I50 vs. resistance effective concentration (REC I50) gave a linear fit with a correlation coefficient (r) of 0.98; an r of 0.98 indicates a high positive relationship between the ability of these fasciolicides to uncouple oxidative phosphorylation and their ability to lower electrical resistance. These findings are consistent with the view that TCZ and its sulfoxide and sulfone metabolites are lipophilic protonophoric uncouplers of rat liver mitochondrial oxidative phosphorylation.