The use of the wood mouse (Apodemus sylvaticus) as an indicator species to assess the effects of chlorfenvinphos and an organomercurial fungicide used as seed dressings on winter wheat have been investigated. Optimum assay conditions for measurement of glutamate oxaloacetate transaminase, glutamate pyruvate transaminase, glutamate dehydrogenase, and sorbitol dehydrogenase in rodent plasma were developed for this study. The amount of surface grain and the chlorfen vinphos and total mercury residues on the surface grain declined with time after drilling. Wood mice were captured using a grid of small mammal traps placed over the surface of a field sown with treated winter wheat and also in adjacent woodland. No trend in mouse body total mercury was discernible after sowing unlike mouse gut chlorfenvinphos which declined with time. Mice trapped in the wood contained very low levels of chlorfenvinphos and total mercury unlike those mice trapped on the field. Plasma acetylcholinesterase and cholinesterase showed significant inhibition 10 days after drilling and liver nitrophenyl acetate esterase showed significant inhibition during the first 3 days after drilling in mice trapped on the field. This inhibition of esterase activities reflected the chlorfenvinphos residue in the gut contents of the mice. The enzyme changes further differentiated mice trapped on the field from those caught in the wood.