In the present study, the distribution and nature of esterases in the rat which hydrolysed fluazifop-butyl, carbaryl, paraoxon and phenylacetate were investigated. Vmax and Km values for the hydrolysis reactions were determined. Fluazifop-butyl was hydrolysed to fluazifop by rat liver (Vmax mumol/min/g microsomes 6.2 +/- 0.4; cytosol 6.84 +/- 0.85), lung (Vmax microsomes 0.38 +/- 0.1; cytosol 1.5 +/- 0.32) and skin (Vmax microsomes 0.02 +/- 0.0015; cytosol 0.4 +/- 0.06) and by plasma (Vmax mumol/min/mL 5.8 +/- 0.48) and red blood cells (Vmax 0.03 +/- 0.015). Significant inhibition by paraoxon and bismitrophenol phosphate indicated the involvement of carboxylesterases. Carbaryl was hydrolysed by liver, lung and skin at a lower rate by microsomal fractions (Vmax nmol/min/g 2.1 +/- 0.25, 1.6 +/- 0.25, 0.2 +/- 0.035, respectively) compared to cytosolic fractions (Vmax 6.7 +/- 0.75, 1.4 +/- 0.36, 0.5 +/- 0.12) and plasma (Vmax nmol/min/mL 3.0 +/- 0.25). Hydrolysis involved carboxylesterases. Paraoxon was hydrolysed by paraoxonases/arylesterases only in the plasma (Vmax nmol/min/mL 246 +/- 12) and microsomal fractions from liver (Vmax 330 nmol/min/g +/- 25) and lung (Vmax 2 +/- 0.25). Phenylacetate was hydrolysed by both microsomal and cytosolic fractions from all tissues studied. Hydrolysis involved arylesterases in the microsomes and carboxylesterases in the cytosol. Extrahepatic hydrolysis may be important following some routes of exposure to xenobiotic esters.