Body water deficits or hypohydration (HYP) may degrade cognitive performance during heat exposure and perhaps temperate conditions. Cold exposure often induces HYP, but the combined effects of cold and HYP on cognitive performance are unknown. This study investigated whether HYP degrades cognitive performance during cold exposure and if physical exercise could mitigate any cold-induced performance decline. On four occasions, eight volunteers completed one hour of militarily-relevant cognitive testing: 30 min of simulated sentry duty/marksmanship, 20 min of a visual vigilance task, a self-report workload assessment, and a mood questionnaire. Testing was conducted in a cold (2 degrees C) or temperate (20 degrees C) environment before and after cycle ergometer (60 min at 60% of VO(2peak)) exercise. Each trial was preceded by 3 h of passive heat stress (45 degrees C) in the early morning with (euhydration, EUH) or without (hypohydration, HYP; 3% body mass) fluid replacement followed by prolonged recovery. HYP did not alter any cognitive, psychomotor, or self-report parameter in either environment before or after exercise. Cold exposure increased (p<0.05) target detection latency in the sentry duty task, adversely affected mood and workload ratings, but had no impact on any other cognitive or psychomotor measure. After completing the exercise bout, there were modest improvements in friend-foe discrimination and total response latency in the sentry duty task, but not on any other performance measures. Moderate HYP had no effect on cognitive and psychomotor performance in either environment, cold exposure produced equivocal effects, and aerobic exercise improved some aspects of military task performance.