When fatty fish are transformed into surimi, lipid oxidation takes place, decreasing the quality of the product. This study was aimed to identify the critical stages of the process in terms of the development of lipid oxidation. Horse mackerels were transformed into surimi on a pilot line and samples taken (hand-skinned fillets = minced fillets, mince, washed and refined minces, paste, surimi and washing water). Most of the lipids were removed during the process and neutral lipids were lost in higher proportion than polar lipids. As a consequence, total lipids of surimi contained more polyunsaturated fatty acids (338 19 g kg(-1)) than total lipids of the minced fillets (220 8 g kg-1). Thiobarbituric acid reactive substances (TBARS) was higher in the minced fillets than in the mince because less subcutaneous fat and dark muscle were removed during hand-mincing, indicating that the settings of the skinning-deboning machine can strongly influence the final quality of the product. Concentrations of lipid oxidation products increased significantly during the next stages of surimi processing. The increase was more pronounced for TBARS than hydroperoxides. Concentrations in hydroperoxides were similar in mince and washed mince (15.3 +/- 2.8 and 16.6 +/- 2.8 mmoles kg(-1) lipid) and increased in refined mince (29.6 +/- 2.8 mmoles kg(-1) lipid). TBARS accounted for 2.7 +/- 1.0 mg kg(-1) lipid in mince, 40.4 +/- 2.3 mg kg(-1) Rpid in washed mince and 237 +/- 7 mgkg(-1) lipid in refined mince. Hydroperoxides and TBARS were found in appreciable amounts in washing water (76.9 +/- 4.7 mmoles kg(-1) lipid and 479 +/- 8 mg kg(-1) lipid respectively), when they decreased in surimi (27.3 +/- 3.8 mmoles kg(-1) lipid and 44.2 +/- 0.8 mg kg(-1) lipid respectively) compared with refined mince. This shows that the last dewatering stage is crucial to ensure surimi quality.