The distribution of lipids in the flesh of large size brown trout (mean body weight 3.8+/-0.2 kg) was analyzed using magnetic resonance imaging (MRI) scanning. Assays were conducted with fish from a control line and from a line selected for growth rate, both fed a low- or a high-energy diet for 4 months. The MRI acquisition sequence used was 15 ms echo time (TE), 140 ms repetition time (TR) and 90degrees flip angle. Based on preliminary complete scanning of one fish, 15 different sections were defined in three areas designated by external anatomical references. The images were corrected for different scanning biases, and the voxel units were calibrated for lipid content of the flesh using external references. Values obtained through MRI matched well with those obtained by chemical and NMR analyses of lipid content (r(2)>0.91). Automated image analysis was conducted to calculate lipid content in the different muscle regions. Wide differences were observed, demonstrating both anteroposterior and central to periphery gradients. These differences are discussed in relation to tissue distribution, muscle activity and energy requirement for locomotion. There was no difference in lipid content or localization between the two lines. Feeding a high-energy diet resulted in significantly higher lipid deposition, especially in the anterior region of the muscle. (C) 2004 Elsevier B.V. All rights reserved.