The aim of this study was to examine the effects of heat stress (HS) on the pituitary-adrenal axis and whether the treatments with early-age thermal conditioning (ETC) and vitamin C, alone and in combination, could have a beneficial effect in alleviating these effects. For the experiment, 400 one day-old broilers both sexes were used, which were divided into 4 groups. The first group was the control (K); the second group (C) were broilers which received vitamin C from the 22nd to the 42nd day via water in the amount of 2 g/L. In the third (T) group, broilers were exposed to ETC for a period of 24 h at a temperature of 38 ± 1ºC, and the fourth group (T+C) was the combination of T and C groups. Immunohistochemically positive adrenocorticotropic hormone (ACTH) cells of broilers in all groups were irregular or stellate and distributed in the periphery and central parts of the pituitary gland, as solitary cells or in clusters. In the T+C group of broilers, we observed a significant (p<0.05) increase in the area of ACTH cells (18.91%), their cores (22.75%) and cortisol in serum concentration, compared to the control group. This reaction of broilers in the T+C group facilitated their adaptation to unfavorable consequences of HS. These results suggest that HPA axis is stimulated after the exposure to chronic heat stress, which enables successful adaptation of broilers to adverse conditions.