Veterinary medicines are administered to animals to treat disease and protect their health. After administration, the substances can be metabolised and a mixture of the parent compound and metabolites may be excreted in the urine and faeces. For animals on pasture, the excreta will be released directly to soil whereas for intensively reared animals, the main route of entry will be through slurry and manure spreading. Whilst the behaviour of other classes of substance (e.g. pesticides and nutrients) that are applied to soil is well understood, limited information is available on the transport and fate of veterinary medicines applied to soils. Laboratory and field studies were, therefore, performed to investigate the sorption behaviour of the sulfonamide antibiotic, sulfachloropyridazine, in soil and to assess the potential for sulfachloropyridazine to move from soil to surface waters and groundwaters. Sorption coefficients (K(D)) for the compound in soil and soil/slurry mixtures were low (ranging from 0.9 to 1.8 l kg(-1)) and indicated that the substance would be highly mobile. Field studies on a clay field supported these observations and demonstrated that, after application, the compound was rapidly transported to surface waters, concentrations of up to 590 microg l(-1) being observed in drainage waters. Leaching studies at a sandy site indicated that the substance had a low potential to leach to groundwaters, concentrations in the soil pore water being below or close to analytical detection limits. An assessment of currently available models for predicting concentrations of veterinary medicines entering surface waters indicated that for sulfachloropyridazine, the methods provide reasonable estimates, predicted concentrations being within a factor of two of the maximum measured concentrations. The approaches may not, however, be appropriate for use on highly hydrophobic substances or for predicting groundwater concentrations.