Formic acid (850 g/kg) was added to wilted perennial ryegrass (dry matter = 360 g/kg) at the rate of 3.9 g/kg before ensiling. The feeding value of the resulting silage was compared with those of silages made from the wilted grass and the freshly cut grass, and with the grass itself in metabolism trials with sheep. Intakes of dry matter were depressed by fermentation, being 12.3, 9.7 and 8.5 g/kg liveweight for the acid treated, prewilted and directly ensiled material respectively. Fermentation improved the metabolisable energies which were 11.6, 12.0, 11.4 and 13.6 MJ/kg DM for the grass, and the acid treated, wilted and directly ensiled materials. In general, fermentation reduced the intake of metabolisable energy and the production potential of the material. The major factor affecting the feeding value of silage is its acceptability to the animal in terms of dry matter intake. High dry matter intakes are best achieved by limiting fermentation.