Anthelmintics and endectocides used for the treatment and prophylaxis of Ostertagia sp. in ruminants include benzimidazoles, levamisole, morantel and the avermectins and milbemycins. Most of these agents are excreted to some extent in the faeces of treated animals and it has been demonstrated that members of the avermectin/milbemycin group may have deleterious effects on non-target organisms utilising the faeces. The environmental impact of antiparasitic chemotherapy depends on the deleterious effect which the agent or its metabolites have on organisms in the locus of the excreta. the amount of active agent excreted, the temporal nature of the excretion and the stability of the ecotoxic residues. These have to be considered in the context of the overall proportion of excreted faeces from a herd which is contaminated and thus the availability of non-contaminated faeces which may act as refugia for dung utilising organisms. The contribution which weathering, faunal inhabitants, trampling by cattle and disturbance by birds have on the rate of dung degradation must also be considered. The greatest ecotoxicological risk is associated with sustained release delivery devices, delivering endectocides with potent activity against dipteran flies and coleopteran beetles. The relatively large proportion of most cattle herds excreting faeces with no endectocidal contamination is likely to reduce the impact that such treatment or prophylactic strategies have on non-target organisms. (C) 1997 Elsevier Science B.V.