Abstract: A study of the effects of high environmental temperatures on the packed cell volume of Thai Indigenous Chickens (TIC), Thai Indigenous Chickens Crossbred (TICC) and Broilers (BC) was made and a comparison of the effects to the 3 breeds was undertaken. One kilogram of representative males and females of each of the 3 breeds were maintained in the environmental temperature range of 26±2 and 38±2�C. Packed cell volumes were investigated on day 1, 7, 14, 21 and 28 of an experimental period. The results revealed the following information: On day 1, the packed cell volume of chickens maintained at 26±2�C was significantly lower than that of chickens at 38±2�C (p< 0.05). On day 7, 14, 21 and 28, the packed cell volume of chickens maintained at 26±2�C was significantly higher than that of chickens at 38±2�C (p< 0.05). On day 7, 14, 21 and 28 of experimental period, at 38±2�C, the packed cell volume of the TIC was significantly higher than that of the TICC and BC (p< 0.05). This experiment showed that the TIC was more tolerant to high environmental temperatures than the TICC and BC.