We studied the effects of different CHO supplements on exercise metabolism (1 hr at 75% VO2max) and performance (fatigue time at 85% VO2max) in 8 male endurance athletes (VO2max = 68.8 +/- 3.8 ml.kg-1.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% VO2max (increases 24%) 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, VO2, 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+] compared to placebo.