The present study was performed to investigate the clinicopathological changes associated with exposure to heat stress in chickens. One hundred Hubbard chickens were used in this study. Experimental chicks were divided into five equal groups each containing 20 chickens. The first gp. chickens were kept as negative control (non treated non heat treated), gp. 2 chickens were exposed to 42±1°C for 3 hrs. with RH. 45-60% at 45 days old, gp. 3 chickens were administrated with No Heat-S with a dose 1 gm/liter in drinking water 24hrs. before heat exposure and exposed to 42±1°C for 3hrs with RH. 45-60%. at 45 days old, gp. 4 chickens were exposed to 39±1°C for 6 hrs. for 3 successive days with RH. 45-60% at 43, 44 and 45 days old while gp. 5 chickens were administrated with No Heat-S with a dose 1 gm/liter in drinking 24hrs. before heat exposure and exposed to 39±1°C for 6 hrs. for 3 successive days with RH. 45-60%. at 43, 44 and 45 days old.
Blood gases, acid/base balance and electrolytes revealed respiratory alkalosis and hyper capnia in heat treated chickens with disturbance in electrolyte immediately after exposure. After 12 hrs., the alkalosis persisted in acute heat treated group only with increase the bicarbonate level in all groups. Hypo capnia occur in all heat treated gps. The hyper kalemia disappeared. After 36 hrs., no changes in blood PH and blood gases were persisted. 
Regarding the biochemical changes, Serum and liver ascorbic acid decreased significantly in case of acute and chronic non treated groups retuned to normal levels 36 after exposure. Serum AST and CK activities were significantly increased in birds exposed to acute and chronic heat exposure which persisted 12 hrs. after exposure and returned to normal 36 hrs. after exposure, hyperuricemia, hypocalcimia and increased creatinine level occurred in birds exposed to acute and chronic heat exposure which persisted 12 hrs. past exposure and returned to normal 36 hrs. after exposure.