On effects of various organophosphorus insecticides such as chlorfenvinphos (phosphate type), EPN (phosphonothionate type), fenitrothion (phosphorothionate type) and mecarbam (phosphorothiolothionate type), and carbaryl (carbamate) on serum pseudocholinesterase, serum nonspecific esterase, serum lipoprotein lipase, adipose tissue lipase and epinephrineinduced lipolysis, this investigation was caried out in vitro. The results were as follows; 1. These insecticides caused a dose-dependent inhibition of serum nonspecific esterase activity as well as serum pseudocholinesterase. Chlorfenvinphos exhibited the strongest inhibition of serum pseudocholinesterase, but severe inhibition of serum nonspecific esterase was recognized carbaryl rather than chlorfenvinphos. 2. Carbaryl and fenitrothion caused slight stimulation of the serum lipoprotein lipase activity with addition of 10-5 M, but inhibited this enzyme with 10-3 M of these insecticides 3. Addition of 10-3 M of these insecticides caused inhibition of adipose tissue lipase in lipolytic system using ediol as substrate. These insecticides also inhibited epinephrineinduced lipolysis as well as inhibition of the adipose tissue lipase. 4. On the base of these findings, it was suggested that these insecticides caused the inhibition of lipolysis by direct suppression of adipose tissue lipase without reference to cAMP dependent proteinkinase.