AbstractFish viscera silage has been prepared from cod (Gadus morhua) and saithe (Pollachius virens) by mincing the viscera and adding a mixture of formic and propionic acids [1:1 (w/v)] to a final concentration of 1.5% (w/v). Feeding experiments were performed with rats using: (a) freshly prepared silage; (b) fish viscera silage stored for up to 4 days at 30°C; and (c) de‐oiled silage prepared after autolysis and subsequent storage for up to 60 days at 15°C. The nutritional value of the silage has been improved by storage and lipid removal, giving an increase of net protein utilisation (NPU) from below 60% for freshly prepared silage to 70% or above after lipid removal. This increase of NPU can be attributed mainly to an increase in the level of lysine.