The heart rate response to exercise was studied in 17 normal subjects before, during and after a 21-day Himalayan trek to 5490 m. The group were fitter before the trek when compared to normal subjects of a similar age (P less than 0.001) having a lower heart rate response to exercise for their lean body mass. The trek significantly increased the 'fitness' of the group as a whole (P less than 0.025) but this was not seen in the 5 members of the group who had severe acute mountain sickness. Similar changes were noted within the first 8 days of the trek before symptoms of mountain sickness occurred. The group had normal pulmonary function before the trek but peak expiratory flow rate and vital capacity decreased with altitude. The effect was more marked in the subjects with acute mountain sickness.