The objective of this experiment was to evaluate the ameliorative effect of L-ascorbic acid (AA) and a-lipoic acid (ALA) on tissue damage caused by heat stress (HS) in the liver and kidneys of broilers, and to correlate this effect with the secretion of the heat shock protein-70 (HSP-70) protein and apoptosis. The study broilers of breed Ross 308 were used. Animals included in the control were divided into four groups. Group TN; control thermoneutral broilers (24 degrees C) were not administered with antioxidants in the diet and not subjected to heat stress (TN). Group HS; broilers not administered with antioxidants in the diet but subjected to heat stress (34 degrees C) (HS). Group HS+AA; broilers subjected to heat stress (34 degrees C) and administered with ascorbic acide (AA). HS+ALA; broilers subjected to heat stress (34 degrees C) and administered with alpha- lipoic acid (AAA). Then, all of the groups were sacrificed and organs (liver and kidney) were collected for histopathological and immunohistochemstry evaluations. Histopathological examination demonstrated that, in the liver and kidneys in Group HS, the blood vessels were very hyperemic. Hepatocytes as well as in tubulo epithelial cells of kidney displayed degenerative alterations. Lesions similar to those observed in Group HS were also observed in Groups HS+AA and HS+ALA. However, the liver and kidney HSP-70 secration were greater in Groups HS+AA, HS+ALA and TN than in Groups HS. The greatest number of apoptotic cells was determined in Group HS, whilst the lowest numbers were determined in Groups TN and HS+ALA. Dietary supplementation with ALA was more effective than AA at preventing tissue damage.