Mountain walking (trekking) is a popular activity and is associated with large increases in energy expenditure (Ainslie et al., 2002 J. Appl. Physiol. 93:714-723) similarly trekking has been identified as having cardiovascular health benefits (Kang, 2014 J. Ex. Rehab. 10:225-229). The aim of the present study was to investigate any physiological changes associated with a 6-day mountain walk. Over a 12-year period 134 male (age, 21.0±1.4 years; height 1.80±0.08 m; body mass 78.7±11.3 kg) and 124 female undergraduates (age, 20.8±1.6 years; height 1.67±0.06 m; body mass 63.5±10.3 kg) participated in a 6-day mountain trek. Height was measured using a wall-mounted stadiometer (Seca LS225) in minimal clothing and bare feet. Body mass and composition were measured using a free-standing body composition segmental analyser (Tanita, BC-418). The initial gradient for the submaximum treadmill test was 0%, for 4 minutes. At the end of four minutes gradient increased by 4% (to 4, 8, 12, and 16% respectively) at constant speed 1.5 m s-1 (3.3 mph). After a 30-min rest period, the maximal treadmill test took place (1.5 m s-1 with gradient increasing 2% very minute from 0 to 20%; thereafter the speed of the treadmill increased 0.13 m s-1 (0.3 mph) every minute until volitional exhaustion). During submaximal and maximal exercise oxygen uptake (VO2) was measured using a breath-by-breath system (Cortex Metalyzer) and heart rate was measured by Polar telemetry. The same measures were made at the same time of day after trek completion. The mountain walk was part of an optional module which had received approval from Oxford Brookes University Ethics Committee (UK). Participants were treated in accordance with the principles laid down in the Declaration of Helsinki (1986). Two way mixed ANOVAs were conducted using SPSS (Version 27) followed by a post hoc Bonferroni correction to investigate the influence of sex on submaximal and maximal measures during mountain walking. Differences between men and women were analysed using an independent samples t-test with an alpha set at p<0.05. Post-trek body mass was lower than pre-trek in both men 78.7±11.3 vs. 77.6±10.9 and women 63.5±10.3 vs. 62.6±9.5 kg. There was also a significant reduction in body fat 14.3±5.1 vs. 12.9±4.8%, men and 26.5±7.0 vs. 25.5±6.7%, women (p<0.001). Heart rate was significantly lowered in men (mean: 128±15 vs. 119±12) and women (mean: 142±6 vs. 137±10 b min-1) at the same treadmill gradients after the trek. Oxygen pulse was significantly higher after the 6-day mountain walk 0.179±0.023 (pre-) vs. 0.185±0.026 ml kg-1 b-1 (post-) (mean for all treadmill gradients). The ventilatory equivalent for oxygen (VE/VO2) was significantly higher after the 6-day mountain walk 22.9±4.2 (pre-) vs. 23.8±4.2 (post-) (mean for all treadmill gradients) (p<0.001). There was a significant, although modest, increase in the maximal aerobic fitness (VO2max values) for men (2.7%) and women (2.9%). The present study suggests that breathing efficiency and oxygen delivery during submaximum exercise was improved together with a lowered heart rate and favourable changes in body composition and an enhanced maximal aerobic fitness as a result of a 6-day mountain walk.