The use of veterinary drugs and zootechnical feed additives has become widespread in modern agricultural practice. Each Member State of the European Union is required to monitor for residues of these compounds in animal tissue and animal products that are intended for human consumption. From 1st September, 1999, the use of the zootechnical feed additives, carbadox and olaquindox is not permitted within the European Union. An electrospray LC-MS-MS method for the confirmation of carbadox and olaquindox in meal is described. The method was developed for contamination level analysis of each drug in feed. An electrospray LC-MS-MS method for the carbadox marker residue in tissue, quinoxaline-2-carboxylic acid, is described. The method was validated over the range 3.0 - 300 pg kg"1, to reflect the range of concentrations measured in NI National Surveillance Scheme samples. The Decision Limit and Detection Capability are also described. This tissue method was applied to the pharmacokinetic study of pigs exposed to housing contaminated by carbadox treated animals. Tentative criteria are suggested for a method that enables National Authorities to discriminate between deliberate use and accidental exposure to contaminated housing. A dual electrospray LC-MS-MS method for the carbadox and olaquindox tissue marker residues, quinoxaline-2-carboxylic acid and methyl-3-(quinoxaline-2)-carboxylic acid, respectively, are presented. The method was validated over the range 3.0 - 150 pg kg"1, to reflect the range of possible concentrations to be measured in Northern Ireland National Surveillance Scheme samples. The Decision Limit and Detection Capability are also described. All the presented methods have been validated according to current European Union criteria for veterinary residue analysis (European Commission Decision 2002/657/EC). All the described methods are to be applied to the statutory analysis of samples taken in Northern Ireland.