Violative chlortetracycline (CTC) residues in excess of the maximum residue limit (MRL) have been identified in 0.9% of all pigs tested at slaughter in Northern Ireland. Three experiments were carried out to investigate the possible reasons. In Experiment 1, pigs were fed CTC at therapeutic levels (300 mg kg-1 head-1 d-1) for 7 days and were slaughtered at day 0, 2, 4, 6, 8 and 10 following CTC withdrawal. Muscle, liver and kidney cortex and medulla samples were analysed for CTC residues by using high-performance liquid chromatography (HPLC). In all tissues, residues were less than the MRL from day 2 onwards. Mean CTC concentrations in kidney cortex were greater than medulla concentrations (p < 0.05). In Experiment 2, feed spiked with a sub-therapeutic dose of CTC (40 mg kg-1) was fed to six pigs that were slaughtered after 4, 8 and 12 days of medication. Violative CTC residues were not detected. In Experiment 3, short term supra-therapeutic administration was examined. Eight pigs received 500 mg kg-1 CTC head-1 d-1 for 1 or 2 days and were killed in pairs, 24 and 48 h following the final medicated feed. Violative levels were detected in both groups after 24 h only. It is concluded that: firstly, recommended CTC withdrawal periods are sufficient to avoid violative carcass residues; secondly, violative residues do not arise from sub-therapeutic dosing but can arise from supra-therapeutic dosing which could occur with inadequately mixed feed; and thirdly, the difference in kidney cortex and medullary CTC concentrations is significant from a regulatory perspective.