During two successive trials interactions between forages and concentrates were studied in sheep. The first paper gives the results concerning feed intake and above all forage/concentrate substitution rates. The following factors of variation were examined: type and proportion of concentrates, type of forage (here only hay). Five concentrates based on soyabean hulls, wheat-bran, cereals, beet-pulp and lupine were used in the first trial and a combination of 3 hays and 2 concentrates (barley and beet-pulp) in the second one. The chemical composition of feeds is given. Each diet combining one hay and one concentrate was given ad libitum to 12 sheep. These diets were calculated so as to supply 0, 30, 60 and 90%, respectively of concentrates in the diet (3 sheep for each level of concentrates). Substitution rates were always calculated by comparison with hay given alone; results are reported in table 2 (variations from 0.34 to 2.08). Moreover, some of these diets were offered to fistulated sheep to measure the cellulolytic activity, VFA, pH and level of NH3 in the rumen. The substitution rate increased systematically with the proportion of concentrates in the diet, but this increase was high with cereal based concentrates and low with the other concentrates (beet-pulp, lupine, soyabean hulls). For a low proportion of concentrates, the substitution rate was lower or the same for cereals as for concentrates with a large proportion of cell-wall constituents (beet-pulp, soyabean hulls). For a high proportion of concentrates, the substitution rate was always higher with starch based concentrates. Increase in the dietary crude protein level reduced the substitution rate. The substitution rate was also affected by the type of forage used. It was the highest with the most ingestible hay. Thus, the forage/concentrate substitution rate was highly variable. This was primarily due to variations in the bulk effect of the combined feeds. Indeed, on account of the physical regulation of appetite, the forage was merely replaced by the concentrate when the proportion of the latter was small. The concentrate was generally digested more rapidly and the substitution rate did not exceed 1. Increase in the proportion of concentrates led to a decrease in the cellulolytic activity in the rumen and therefore in the digestion rate of the forage. This led to an increase in the bulk effect of the latter and accordingly in the substitution rate. However, there were large variations due to the type of feeds used and to the effect of a metabolic regulation of feed intake, more marked for starch containing concentrates.