Esterase activity was investigated in several skeletal muscles of different rodent species employing as substrates a series of .alpha. and .beta. naphthyl esters of different aliphatic chain lengths. Results with these chemically homogeneous substrates show there is marked heterogeneity in the levels of esterase activity in several skeletal muscles of the same animal and among the same muscles from different species depending also on the sex, and the type and stereospecificity of the substrate.