Whether mental performance is affected by slowly progressive moderate dehydration induced by water deprivation affects mental performance has not been examined previously. Therefore, objective and subjective cognitive-motor function was examined in 16 volunteers (8 females, 8 males, mean age: 26 yrs) twice, once after 24 h of water deprivation and once during normal water intake (randomized cross-over design; 7 d interval). Water deprivation resulted in a 2.6% decrease in body weight. Neither cognitive-motor function estimated by a paced auditory serial addition task, an adaptive 5-choice reaction time test, a manual tracking test, and a Stroop word-color conflict test nor neurophysiological function assessed by auditory event-related potentials P300 (oddball paradigm) differed (p>0.1) between the water deprivation and the control study. However, subjective ratings of mental performance changed significantly towards increased tiredness (+1.0 points) and reduced alertness (-0.9 points on a 5 point scale; both: p<0.05), and higher levels of perceived effort (+27 mm) and concentration (+28mm on a 100mm-scale; both: p<0.05) necessary for test accomplishment during dehydration. Several reaction time-based responses revealed significant interactions between gender and dehydration, with prolonged reaction time in women but shortened in men after water deprivation (Stroop word-color conflict test, reaction time in women: +26ms, in men: -36ms, p<0.01; paced auditory serial addition task, reaction time in women +58ms, in men -31ms, p=0.05. In conclusion, cognitive-motor function is preserved during water deprivation in young humans up to a moderate dehydration level of 2.6% of body weight. Sexual dimorphism for reaction time-based performance is present. Increased subjective taskrelated effort suggests that healthy volunteers exhibit cognitive compensating mechanisms for increased tiredness and reduced alertness during slowly progressive moderate dehydration.