The objective of this experiment was to compare the ability to tolerate heat between Thai Indigenous (TIC), crossbred Thai Indigenous (TICC) and commercial Broiler Chickens (BC). A Split-plot completely randomized design was used. Twenty four TIC, 24 TICC and 24 BC (12 males; 12 females), each 1 kg of weight and infectious disease-free, were obtained from a commercial farm near MahasarakhamUniversity. The 3 breeds were maintained at 26±2°C (continuous temperature) and 38±2°C (cyclic temperature) for 28 days. Their pathological changes were observed. Lesion scores, according to pathological changes were determined under microscopic examination and compared between breeds. The results revealed the following information: generalized edema and hemorrhage in the kidney, especially in the renal pelvis and renal tubular epithelia was observed. A space in the renal tubular increased with water accumulation, which might cause renal edema. The glomeruli were damaged. Most of the liver cells in all chickens showed fatty degeneration by vacuolation with dilation of sinusoids. Lesions in the lung were related to the veins and massive congestion of the veins and arterioles was noted. Massive hemorrhage was mainly observed in the parabronchus, alveolar ducts and the alveolar sac. In the cardiac muscle, massive myofibrillar degeneration with haemorrhage and in some cases generalized and diffuse myocarditis containing organisms, was observed. When broiler chickens were maintained at 38±2°C, the severity of pathologic lesions was higher than in the crossbred Thai ingenuous and the Thai ingenuous chicken (p<0.05). © Medwell Journals, 2009.