Three-day event horses are subject to various external environmental stresses including changes in ambient temperature, humidity, altitude, and test severity. Considerable research on the adverse effects of increased heat and humidity preceded the 1996 Olympic Summer Games in Atlanta, Georgia USA, but no research has been done previously on the effects of altitude on 3-day eventing. Physical and venous blood gas data were collected on horses (n = 24) competing in the High Prairie Preliminary (CCN*) and Intermediate (CCN**) 3-day events and Preliminary Horse Trials in Parker, Colorado (1900 m above sea level). Despite the increased altitude, only post exercise rectal temperature and pH were higher (P<.05) whereas heart rate (HR), [K+], and ionised calcium (iCa++) were lower (P<.05) in 3-day event horses compared to horse trial horses. All other variables (respiratory rate [RR], PCV, [Hb], Pco2, [tco2], [HCO3-], BE, and [Na+]) were not different between groups (P>0.05). When these preliminary horse trial horses in Colorado were compared to those previously studied at preliminary horse trials at sea level in Arizona, post exercise HR and RR were higher (P<.05) and pH, Pco2, [tco2], [HCO3-], BE and [iCa++] were lower (P<.05) at altitude. These data show that increased altitude (1900 m above sea level) was more stressful for 3-day event horses, but did not result in the severe physiological changes and inability to complete prescribed exercise tests seen in previous studies with increased heat and humidity. It is clear from these and previous data that increased heat and humidity are the more important environmental stressors in 3-day eventing.