TWO pig experiments and two rat experiments were conducted to compare the effects of supplemental lysine in drinking water with equivalent amounts of lysine in low protein dry diets on the performance of the animals. In experiment 1, pigs fed lysine in feed or water had similar rates of gain, feed:gain ratios and carcass measurements. Lysine supplementation was most beneficial in the diet when the pigs were smaller by supporting faster gains with less feed per unit of gain.In experiment 2, paired pigs fed an equal quantity of feed and an equal amount of lysine via water or feed had essentially the same rates of gain and feed utilization.Lysine in feed or water supported more total gain and better feed utilization of rats which were fed a low protein diet. Lysine appeared to be adequate in a diet with 14% crude protein and supplemental lysine was of no benefit. Lysine was effective in both feed and water.