A study was made of the influence of the composition of raw, cooked and canned white tuna fish on the utilisation of nitrogen, calcium and phosphorus in the diet. The fish was canned using industrial methods. The biological tests were carried out on Wistar rats fed for 12 days on semi-synthetic diets which differed only in the protein source: raw tuna, steamed tuna, tuna cooked in brine and canned tuna both freshly prepared and stored for 1 and 3 years. The efficiency of the digestive and metabolic utilisation of the nitrogen, calcium and phosphorus in the diets including raw and steamed white tuna and tuna cooked in brine was similar to that of these elements in the control diet in which the protein source was casein-DL methionine; in addition, the animals ingesting these diets showed an adequate body weight evolution. After canning, there was a slight drop in protein quality and a further drop was noted after storage of the cans at atmospheric temperature for 1 year. This storage period produced no changes in the tuna which were likely to affect calcium bioavailability when both formed part of the same diet but the utilisation of phosphorus decreased notably, with the balance dropping by half. Also the final weights of the animals fed on these diets, which included tuna preserves for 1 year, were significantly lower. After 2 further years of storage of the cans, nutritional utilization of their protein rose to its former level as did that of phosphorus in the diets containing the steamed canned tuna (SC3) but only partially in that of the cooked in brine canned tuna (BC3), possibly because with this diet the calcium balance had dropped slightly.