Regularly shaped samples of flesh from a lean fish from temperate waters, cod (Gadus mourhua,) and a fatty fish from tropical waters, hilsa (Hilsa ilisha), were dried to various extents in a mechanical drier at 30 "C. Similar blocks of muscle were subjected to the same temperature conditions but wrapped to prevent dehydration in order to compare the effect of heating without dehydration. The changes in extractable proteins, non-protein nitrogen, bacterial count, pH and, in the case of hilsa, lipid oxidation are reported. The amount of extractable myofibrillar protein decreases markedly during both heating and drying, the rate being faster for cod than for hilsa and the extra effect of drying over heating alone is different in the two species. The bacterial count increases continuously with time in the case of the heated samples but reaches a steady value and then decreases during drying. Non-protein nitrogen and pH changes are similar to the bacterial changes. There is marked lipid oxidation during both heating and drying, the effect being greater in the dried samples. It is concluded that most of the protein changes during drying are due to the effects of heating rather than dehydration.