Twenty Hubbard cockerels each weighing 2.4 kg were surgically fitted with plastic cannulae in the carotid artery and crop. A solution of either tap water (TW), carbonated water (CW), 2% sodium bicarbonate (HCO3-), or 3.5% calcium chloride (CaCl2) with pH of 7.8, 5.2, 8.0, and 7.4, respectively, was infused into the crop at .41 ml X min-1 X kg body weight (BW)-1. Blood pH, carbon dioxide partial pressure (PCO2), and respiratory rate (RR) were measured at 15-min intervals during a 90-min thermoneutral period (25 C) followed by a 90-min heat stress (37 C). Polypnea occurred in all treatments in the 37 C environment. Infusion of HCO3- increased blood pH throughout the thermoneutral and heat stress periods, and CaCl2 infusion decreased blood pH throughout the experimental periods. At thermoneutral temperature, TW and CW infusions did not affect blood pH or PCO2. During heat stress, blood pH increased in TW and CW treatments; change and final pH were significantly lower (P less than .05) for CW- compared with TW-infused birds. Although blood PCO2 decreased in all treatments with the imposition of heat stress, blood PCO2 was significantly greater (P less than .05) in birds infused with HCO3- at the end of the heat-stress period. The results demonstrate that CW treatment produced a more favorable acid-base balance during acute heat stress than TW by reducing blood pH change during thermal polypnea. Also, high levels of NaHCO3 and CaCl2 intake can produce abnormal acid-base equilibrium conditions during the heat stress-induced respiratory alkalosis.