We studied the effects of heat and relative humidity (RH) on plasma volume (PV) and ion responses to submaximal exercise and 60 min recovery in Thoroughbreds. Five horses were exercised at 50% of peak VO2 in cool, dry (CD, T = 22 degrees C, RH = 45-55%), and hot, humid (HH, T = 30-34 degrees C, RH = 80-85%) conditions until a pulmonary artery temperature of 41.5 degrees C was reached. Blood was obtained from the carotid artery. Body mass was measured at rest and after 30 min of recovery. The thermal conditions had no effect on the PV and ion responses during exercise and initial 30 min of recovery. Exercise resulted in an 8.5% decrease in PV within the first 2 min and, in the absence of changes in plasma [Na+] and [Cl-], was responsible for a 150 to 185 mmol decrease in plasma Na+ and Cl- contents. The decrease in PV was responsible for about 50% of the increase in packed cell volume (37% at rest; 51% at 2 min of exercise). Plasma [K+] and K+ content increased rapidly during the first 2 min of exercise. With cessation of exercise, plasma [K+] declined with a half time of about 2 min; recovery of PV and plasma Na+ and Cl- contents occurred with a half time of 10-15 min, with nearly complete recovery by 30 min. In the HD trial, between 30 and 60 min of recovery there were further decreases in PV and ion contents. It is concluded that with this exercise protocol, thermal stress had minimal influence on the rate and magnitude of exercise-induced changes in PV and ion contents. However, thermal stress during recovery from exercise may impede the restoration of PV and ion contents.