We studied the effects of different CHO supplements on exercise metabolism (1 hr at 75% ${\dot{\text{V}}}_{\text{O}}$) and performance (fatigue time at 85% ${\dot{\text{V}}}_{\text{O}}$) in 8 male endurance athletes $(\stackrel{\cdot }{\text{V}}{\text{O}}_2\max =68.8\pm 3.8\text{ ml}\cdot {\text{kg}}^{-1}\cdot {\min }^{-1})$ Four treatments were administered in a randomized, double-blind fashion: Trial A = 3-day pretest, postexercise supplementation (177 kcal [81% carbohydrate, 19% protein] consumed < 10 min after exercise) + 600 ml 8% glucose polymers/ fructose 1 hr pretesting + 600 ml 8% glucose polymers/glucose during testing; Trial B = placebo during 3-day pretest + remainder same as Trial A; Trial C = placebo at all time points; and Trial D = same as Trial B with 8% glucose 1 hr before the test as well as during the test. Time to fatigue at 85% ${\dot{\text{V}}\text{O}2}_{}\text{max}$ (Í24%) and total CHO oxidation were greater for A versus C (p < .05). Plasma glucose concentration was higher for A and B versus C, while increases in plasma potassium concentration were attenuated for A versus C (both p < .05). None of the supplements had differential effects upon hematocrit, plasma sodium [Na⁺] and lactate, ${\dot{\text{V}}\text{O}2}_{}$, or rating of perceived exertion during exercise. Three-day preexercise protein + carbohydrate supplements followed by 1-hr pre- and during-exercise mixed carbohydrate supplements increased time to fatigue and carbohydrate oxidation and attenuated rises in plasma [K⁺] com pared to placebo.