Compared to studies on the cockerel and the domestic laying hen, little information is available concerning the progressive alteration to acid-base balance in broiler chickens exposed to acute heat stress. The primary objective of this study was to determine the time course of changes to core temperature, respiratory rate and blood acid-base parameters of broiler chickens during an episode of acute heat exposure (36 or 38°C). Acute exposure of chickens to 36 and 38°C resulted in significant increases in core temperature, with maximum values reaching 44 and 46°C, respectively. In the first 30min when rectal temperature increased significantly in the 38°C heat-exposed group, a clear and significant concomitant increase in blood pH was also identified. Conversely, pCO₂ and bicarbonate concentration in the heat-exposed animals had decreased significantly 60min after the onset of exposure to heat, but not at the 30min time point. In contrast to previous findings of the time course of changes in arterial pH and pCO₂ of hens exposed to acute heat stress, there was evidence here of a time lag for pCO₂ to decrease after the onset of heat exposure and no compensation for the blood alkalosis at the 90- and 120-min time points after heat exposure. These findings imply the absence of a mechanism for pH compensation in broiler chickens.