AbstractIn this investigation the effects of chlorophenols on rat liver mitochondrial respiratory parameters were determined and compared to the toxicities of those compounds in a variety of biological systems currently being used for toxicity testing. Mitochondrial fractions were exposed to six concentrations of five different chlorophenols in a semiclosed, 2‐ml reaction vessel. Respiratory parameters were measured polarographically with an oxygen electrode and compared to control experiments. The toxicity of the chlorophenols, as measured by the concentration reducing the respiratory control ratio of the control to 50% (RCR50), increased with increasing chloro substitution. RCR50 concentrations ranged from 599 μM with 2‐chlorophenol to 0.110 μM with pentachlorophenol.The RCR50 concentrations for the five chlorophenols were compared to six physicochemical parameters for the same chlorophenols; high degrees of correlation between the RCR50 concentrations and the physicochemical parameters were found (r≥ 0.890). The highest correlation coefficient obtained was with then‐octanol/water partition coefficient (r= 0.991), indicating that the ability of chlorophenols to partition into the lipid mitochondrial membrane plays a significant role in eliciting its toxic effect. RCR50 concentrations were also compared to nine currently existing short‐term toxicity tests. High degrees of correlation were obtained with several of the tests, including algal, bacterial, and fish bioassays. This suggests that the uncoupling of oxidative phosphorylation may be the major mechanism by which chlorophenols cause toxicity to intact cells as well as more complex organisms.The use of mitochondrial respiratory parameters appears to offer a complementary approach as a short‐term toxicity test for this class of compounds. Further development and testing with a variety of other toxicants