This study was conducted to describe histopathologic changes in lymphoid organs of broiler and native chickens in response to heat stress. A total of 60 day-old broiler and native chicks were allocated in 3 treatment groups. The control group (T1) was exposed to ambient environmental temperature (AET); Treatment 2 (T2) was exposed to AET + ≈1°C; and T3 was exposed to AET + ≈3°C. Each treatment was exposed to the required temperature for 7 hours from d 29 to 35 (AET = 29.8°C) and from d 43 to 49 (AET = 31.7°C). Results showed significant microscopic changes in the lymphoid organs that include lymphoid depletion in the Bursa of Fabricius and lymphoid depletion and lymphocytic degeneration in the germinal centers of the spleen. The thymus had minimal to mild degenerative changes mainly in the cell population of the medulla. The results also showed more pronounced microscopic changes in the Bursa of Fabricius and spleen of broiler chicks compared to native chicks. The degree of response to heat stress appears to be breed-dependent with the lymphoid organs of broiler chickens showing lesser adaptive capacity to withstand high temperature than the lymphoid organs of native chickens.