Abstract: The purpose of this experiment was to study the effects of high environmental temperatures on the electrolyte status of three breeds of chickens: Thai Indigenous Chickens (TIC), Thai Indigenous Crossbred Chickens (TICC) and Broiler Chickens (BC). Male and female TIC, TICC and BC were maintained at the environmental temperature ranges of 26±2°C and cyclic 38±2°C. Sodium (Na+), chloride (Cl¯) and potassium (K+) were investigated on days 1, 7, 14, 21 and 28 of the experimental period. The results revealed the following information: On day 7, plasma Na+ of male BC at 38±2°C was significantly higher than that of male BC at 26±2 °C (p<0.05). On day 14, plasma Na+ of male TIC and BC at 26±2°C was significantly higher than that of male TIC and BC at 38±2°C (p<0.05). On day 28, plasma Cl¯ of male TIC and BC at 38±2°C was significantly higher than that of male TIC and BC at 26±2°C (p<0.05). On day 28, at 38±2°C, plasma Cl¯ of male TIC was significantly higher than male TICC and male and female BC (p<0.05). On days 1, plasma K+ of TIC, TICC and BC at 38±2°C was significantly higher than that of TIC, TICC and BC at 26±2°C (p<0.05). On day 7, plasma K+ of TIC and TICC at 38±2°C was significantly higher than that of TIC and TICC at 26±2°C (p<0.05). On day 14, plasma K+ of male BC at 38±2°C was significantly lower than that of male BC at 26±2°C (p<0.05). On days 21, plasma K+ of female TICC at 38±2°C was higher than female TICC at 26±2°C (p<0.05). On day 28, plasma K+ of male TIC and TICC at 38±2°C was significantly higher than that of TIC and TICC at 26±2°C (p<0.05). This experiment showed that time, breeding and environmental temperature have an influence on the electrolyte status of TIC, TICC and BC. Under heat stress, TIC preserved the electrolyte in their body better than the TICC and BC, respectively.