1. Six unacclimatized African mine labourers were subjected to exercise for 4(1/2) hr in a hot humid environment (90 degrees - 93 degrees F wet-bulb/dry-bulb (W.B./D.B.); approximately 90% r.h.) 2. The patterns of glandular activity and the densities of active glands on the chest and back were assessed half-hourly from plastic impressions. Acclimatization increased and prolonged glandular activity. The increment in activity of the sweat glands on the back was greater than that on the chest. 4. There was no significant increase in the maximum number of active glands on either site after acclimatization. 5. Acclimatization greatly reduced the number of inactive glands, subsequent to the maximum count, on the back, but this was not observed on the chest. 6. The increased sweat rates with acclimatization were due mainly to increased glandular activity. 7. The decline in sweat rates and activity on prolonged exposure to hot humid environment was attributed to glandular fatigue. Other factors, such as increased body temperature, hydration of the skin and fatigue of the central nervous system, suggested by other investigators as possibly causing the decline in sweat rates, did not have support in this study.