The hydrolysis activity and expression level of carboxylesterase (CES) in skin were compared with liver and intestine in the same individual of beagle dog and cynomolgus monkey, and their aging effects were studied. CES1 isozymes were mainly present in skin of both animals. The dermal hydrolysis activity was about 10 and 40% of hepatic activity in beagle dog and cynomolgus monkey, respectively. In beagle dog, the hydrolysis activity and the expression level of CES isozyme in liver and skin were nearly the same between 2- and 11-year-old individuals. On the other hand, the dermal hydrolase activity was lower in young individual than in old, in contrast to slight increase of hepatic and intestinal activity in old cynomolgus monkey. These differences by aging in cynomolgus monkey were related to the expression of CES1 proteins and their mRNA. Furthermore, mRNA level of human CES was investigated using total RNA of two individuals (63 and 85 years old). The two individuals showed approximately 2-fold higher expression of hCE2 than hCE1 in human skin.