This study examined the complex relationships of fitness and hypoxic sensitivity with submaximal exercise responses and Acute Mountain Sickness (AMS) at altitude. Determining these relationships is necessary before fitness or hypoxic sensitivity tests can be recommended to appraise individuals’ readiness for altitude. Forty-four trekkers (26 men; 18 women; 20-67 years) completed a loaded walking test and a fitness questionnaire in normoxia to measure and estimate sea-level maximal aerobic capacity (V̇O2max), respectively. Participants also completed a hypoxic exercise test to determine hypoxic sensitivity (cardiac, ventilatory, and arterial oxygen saturation responses to acute hypoxia, FiO2=0.112). One month later all participants completed a three-week trek to 5085m with the same ascent profile. On ascent to 5085m, ratings of perceived exertion (RPEascent), fatigue by Brunel Mood Scale, and AMS were recorded daily. At 5085m, RPE during a fixed workload step test (RPEfixed) and step rate during perceptually-regulated exercise (STEPRPE35) were recorded. Greater sea-level V̇O2max was associated with, and predicted, lower sense of effort (RPEascent r=-0.43; p<0.001; RPEfixed; r =-0.69; p<0.001) and higher step rate (STEPRPE35 r=0.62; p<0.01), but not worse AMS (r=0.13; p=0.4) or arterial oxygen desaturation (r=0.07; p=0.7). Lower RPEascent was also associated with better mood, including less fatigue (r=0.57; p<0.001). Hypoxic sensitivity was not associated with, and did not add to the prediction of submaximal exercise responses or AMS. In conclusion, participants with greater sea-level fitness reported less effort during simulated and actual trekking activities, had better mood (less fatigue), and chose a higher step rate during perceptually-regulated exercise, but did not suffer from worse AMS or arterial oxygen desaturation. Simple sea-level fitness tests may be used to aid preparation for high-altitude travel.