Background: The increased threat of nuclear, biological and chemical (NBC) weapons underlines the need of protective clothing and gas masks, but this may impair performance. Thus, attention should be focused on the nutritional requirements. Hypothesis: Optimal performance is guaranteed if the supply of water and energy is adequate. Methods: Two groups of 20 trained military men (mean age 22 yr) received either an isotonic carbohydrate drink or a placebo drink (flavored water) for 24 h under simulated NBC conditions (wearing gas masks and protective clothing). Various physical and mental tests were performed at intervals and blood samples were collected three times. Results: Five men of the placebo group had to be withdrawn during the experiment because of exhaustion. The decrease in physical performance (about 15%) and mental performance (about 20%) was most apparent for the group that had only water for consumption. Conclusions: When only water is consumed, physical performance of a group of military men decreases during 24 h of simulated NBC conditions. An isotonic carbohydrate drink is recommended with respect to maintaining performance under NBC conditions. Energy restriction prior to an NBC scenario has a negative influence on performance.