In the present work an attempt has been made to study the effect of L-thyroxine and thiouracil on thermal tolerance in an air breathing freshwater siluroid teleost, Heteropneustes fossilis (Bloch). LC values of 50 thermal tolerance of this fish (weighing 28.0�1.2 g) was recorded to be 39.5�0.13�C. The L-thyroxine and thiourea treated animals showed significant increase and decrease respectively in their thermal tolerance. The details of the mechanism have been discussed in this paper.