In experiment 1, weanling rats were given a commercial rat diet supplemented with HCl at levels up to 560 mmol.kg-1 dry matter for 7 wk. The supplement increased water intake but did not significantly affect food intake, live-weight gain, blood Hb and hematocrit values or acid-base balance. In experiment 2, adult rats were given a commercial rat diet supplemented with HCl levels up to 1250 mmol.kg-1 dry matter for 9 wk. Food intake and liveweight gain were not affected by HCl concentration up to 625 mmol but at 938 and 1250 mmol.kg-1 they were considerably reduced and there was 100% mortality of the rats. In experiment 3, weanling rats were given, for 12 wk, a commercial rat diet supplemented with HCl or lactic acid each at 300, 600 and 900 mmol.kg-1 dry matter. Lactic acid at the 3 levels and HCl at the 2 lower levels did not affect food intake or liveweight gain and had only a slight effect on blood acid-base balance. At a dietary concentration of 900 mmol.kg-1 dry matter, HCl decreased food intake, induced a mild degree of metabolic acidosis and resulted in 30% mortality of the rats. In the 3 experiments, the acid treatments did not directly affect the length or composition of the femur of the rats. [Acidifying with strong mineral acids was developed as a preservation method for ruminant silage.].