The effects of inhaling dry and humid hot air on expired air relative humidity (RH) during exercise were studied in humans. The study group consisted of nine male volunteers, 18 to 35 years old. They breathed air at 26 degrees-C and 60% RH (cool dry air) or 95% RH (cool humid air) or 45 degrees and 11% RH (hot dry air) or 95% RH (hot humid air) at rest or while exercising at 36 and 70% of maximum aerobic capacity on a cycle ergometer for 50 minutes. Expired air and esophageal temperature, minute ventilation (VE), tidal volume (VT), breathing frequency, oxygen consumption (VO2), carbon-dioxide production (VCO2), heart rate, and mean expired RH (ERH) were monitored. VE, VO2, and VCO2 were not significantly affected by the type of air inhaled. Breathing frequency was significantly decreased and VT was significantly increased at both exercise intensities when hot humid air was breathed compared to breathing cool dry air. Mean expired air temperature was significantly higher when hot dry or hot humid air was breathed compared to when cool dry or humid air was inhaled. Mean ERH was significantly lower across both exercise intensities and rest when hot dry air was breathed compared to the hot humid or cool dry and humid conditions. ERH was decreased during exercise but recovered to the control values regardless of the type of air breathed. Regression analysis showed that mean ERH had a small, but significant dependence on VE. The dependency was more pronounced when hot dry air was breathed. The authors conclude that the study supports the results of other studies which found that ERH depends on several respiratory and environmental factors as well as inspired air temperature.