1. 1. Arylesterase activities were measured with -naphthylpropionate and/or -naphthylacetate as substrate in the plasma of rats, rabbits and humans on low- and high-cholesterol diets. 2. 2. The plasma esterase activities measured with -naphthylacetate were similar in rats, rabbits and humans. With -naphthylpropionate as a substrate, rabbits were found to have a markedly higher esterase activity than rats and humans. 3. 3. Basal plasma esterase activity was significantly higher in an inbred rat strain which is hyporesponsive to dietary cholesterol than in a hyperresponsive strain. 4. 4. In rats, but not in humans and rabbits, plasma esterase activity was significantly increased by a high-cholesterol diet. 5. 5. In individual humans and random-bred rabbits and rats there was no association between initial plasma total esterase activity and the subsequent plasma cholesterol response to cholesterol feeding. 6. 6. We suggest that arylesterases are associated with cholesterol metabolism and with the response to dietary cholesterol in rats; evidence for such a role in rabbits and humans is, however, inconclusive.