A radiochemical assay for carboxylesterase based on the substrate methyl[1-14C]butyrate is described. The blank value corresponds to 1.04 micrograms (liver)-1.44 mg (plasma) of tissues with the highest and lowest activity respectively, which constitute the sensitivity of the method. The hydrolysis of methyl butyrate and 4-nitrophenyl butyrate by plasma, liver, lung, heart, diaphragm, cerebrum, kidney and duodenum of rat have been compared. The results showed that the two substrates were hydrolysed preferentially by two different groups of the enzyme. The effect of selective esterase inhibitors showed that both groups can be characterized as carboxylesterase, because bis-4-nitrophenyl phosphate inhibits the hydrolysis of both substrates, physostigmine has only a slight effect and EDTA is no inhibitor. The exception is the enzyme in the duodenum which is inhibited by all three inhibitors. The effect of phenobarbital induction and soman treatment on enzyme activity towards the two substrates were similar. Sex difference in the plasma activity towards methyl butyrate, but not 4-nitrophenyl butyrate, indicates that the group of carboxylesterase preferentially hydrolyzing 4-nitrophenyl butyrate may be the most important for the detoxification of soman.