Military training prepares leaders for the stress of combat. However, there have been few studies of the response of well-trained officers to brief, but intense, operational stress. Recently, during a continuous 53 hour military field exercise designed to produce severe stress in participating officers, we examined changes in various behavioral and physiological parameters. Participants were Captains in the U.S. Army Rangers with an average of 9 years of military service.  They were exposed to multiple stressors, including minimal preparation time for the exercise, rapid airborne deployment to an unexpected location, high heat and humidity and unanticipated opposing forces (OPFOR) activity. They received minimal rations, were continuously engaged in physical activity and their performance was being evaluated by command authorities. Under these circumstances, their physical status deteriorated, as indicated by a 5% (p < .001) weight loss, consisting primarily of water. Their psychological status, assessed with computer-based cognitive tasks, was severely degraded. Vigilance, reaction time, learning, memory and logical reasoning were impaired (p < .001). Self-reported mood-states such as, confusion (p <.001), fatigue (p < .001), and anger (p = .009) increased dramatically. The performance decrements observed were greater than those observed in individuals with a blood alcohol level of 0.1 %, legally drunk in many localities, or suffering from clinical hypoglycemia.  In spite of such severe physical and cognitive deterioration, cortisol levels of the volunteers indicated they were experiencing minimal stress. The lack of a classic stress response under such severe conditions indicates that extensive prior training, the selection process and perhaps other factors protected these elite officers from the adverse physiological effects of acute stress.