Aims: Free radicals may not only affect the denatured cell structure but also reduce protein levels. These mechanisms are critical and need to be clarified as a high priority. Because of the limited information on this subject, in this respect, the effects on protein levels of liver tissue in rats fed hot-smoked rainbow trout ( Oncorhynchus mykiss) were investigated. Methods: Four diets containing fresh and hot-smoked rainbow trout flesh and vitamins were prepared and commercial pellet food was purchased. Four groups of female Wistar rats were fed the diets for 28 days. The rats were killed by decapitation, and the hepatic tissue was removed. Total protein and detection of protein bands using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) were evaluated. Results: The liver protein level in rats fed a hot-smoked rainbow trout flesh + vitamin diet was increased significantly compared with the other diet groups (p < 0.001, p < 0.009 and p < 0.011). 10 protein bands were visualized on rat gels. The molecular weights of protein bands detected were 200, 130, 70, 50, 42, 38, 26, 25, 24 and 14 kDa, respectively. The 25-kDa band of group V had a dense appearance. Also, the 14-kDa band appeared as very dense in groups V, B and K, but this band did not have a dense appearance in group D. Conclusion: Nourishment with smoked fish diets containing antioxidant substances reduced the harmful effects of free radicals and positive effects were observed on protein metabolism. Copyright (C) 2006 S. Karger AG, Basel.