Four groups of weanling male albino rats (Wistar strain) were fed isonitrogenous diet (10% protein) identical in all respects except in the nature of the protein source, for 4 weeks. Control group (group 1) had steamed mackerel meat as the protein source, whereas groups 2, 3 and 4 had mackerel fried on the 1st, 3rd and 4th days in the same coconut oil repeatedly used for frying each day. Four groups of adult male rats weighing around 130 g were fed on the same diet for 12 weeks. Weanlings fed on fish fried on the 4th day showed significantly lower feed consumption and weight gain compared to the other three groups. All the three groups of adult rats fed on fried fish compared well with control rats in weight gain and hepatosomatic index. There was a decrease in the total lipid and cholesterol content of the liver of rats fed with fried fish in comparison with the control rats. The total lipid and cholesterol in heart and serum cholesterol levels increased significantly from control rats through group 4. The C22:6/C20:5 ratio in the heart lipid showed a very high value compared to the dietary lipids. Histopathological examination showed initial stages of cell damage in the liver and kidney of rats fed with fish fried on the 4th day. In-vitro digestibility of proteins of fried fish were lower than that of steamed fish, but the difference in this respect between proteins of fish fried on different days was not significant.