Broiler chicks were subjected to early feed restriction (EFR) on d3-5 post-hatch to determine the day that confers the best thermal tolerance during acute heat stress episode at market age. In Experiment I, 160 Marshall chicks were allotted to 4 treatments. One group received feed ad libitum (CONTROL), while others received no feed for a period of 24 hours on d3 (D3), d4 (D4) and d5 (D5), respectively. Each group had 4 replicates with a total of 10 birds per replicate. On d55 of age, rectal temperature (RT) was monitored and blood samples were taken from the birds before and 1h after exposure to temperature 37±2oC and 50% relative humidity. There were a total of 120 chicks in Experiment II and they were divided into 3 groups which were either fed ad libitum (CONTROL) or had feed withdrawn for 12h (D512) or 24h (D524) on d5 to determine the length of feed restriction that best conferred thermotolerance on the birds. RT and blood sampling were also done as described above. In Experiment I, haematological parameters were (P>0.05) similar among the treatments before and after exposure. Prior to the exposure to heat stress, RT was not affected (P>0.05) by feeding regimen but it was significantly (P<0.05) affected by the treatment after the exposure. D5 chickens had significantly (P<0.05) lower temperature than control. However, it was not different from D3 and D4 chickens. In Experiment II, initial RT did not differ among the 3 treatment groups although RT was (P<0.05) affected by the feeding regimen after 60 minutes of exposure. The control chickens had similar RT with that of D512. However, RT in control birds (P<0.05) was higher than RT in D524. Thus, applying EFR in broilers on D5 of post-hatch for 24 hours may help reduce hyperthermia during heat spell at market age.