The present study was carried out to examine the effects of heat stress on productive performance and some blood parameters of two turkey breeds namely Commercial White Nicholas (WW) and Black Baladi (BB).
Seventy two birds (36 birds for each breed) were divided equally into three groups (12 birds for each group)  as follows: group A was served as a control and reared at 24 °C±2 , the other two groups (B and C) considered as  heat stressed groups were exposed to 30 °C±2 and 35 °C±2, respectively. Results revealed negative impact of heat stress on productive performance parameters including live body weight, body weight gain, feed consumption and feed conversion ratio which significantly (p≤ 0.01) depressed in both groups (B and C). Effect of breed revealed that White Nicholas significantly (p≤ 0.01) improved the measured productive performance parameters in comparison with Black Baladi. inverse relationships were observed between increasing temperature and both total red blood cells count (TRBC X 10⁶ cells/UL) and hemoglobin concentration (Hb g/dl) which significantly decreased (P≤0. 01) in both heat stress groups (B followed by C). Results of differential leucocyte count revealed significant increase (p≤ 0.01) in basophils percent and lowered lymphocytes associated with high H/L ratios (Heterophil to Lymphocyte ratio) in both B and C groups. Breed effects on TRBC, TWBC and H/L ratio revealed significant decrease in TRBC, WBC and lymphocyte percent with significant (p≤ 0.01) increase in H/L ratio in Black Baladi. Corticosterone concentration shows a high significant (P≤0. 01) increase while T3 and T4 showed a significant decrease (P≤0. 01) in heat stress groups especially in Black breed. therefore, this study concluded that exposure of birds to high temperature (30 and 35°C) leads to depression of productive performance parameters, decreasing TRBC, TWBC percentages , increasing of H/L ratio and corticosterone concentration, and alter body temperature regulating  thyroid hormones. Moreover, turkey breed had a great effect on all the above measured parameter as the Black Baladi breed was more susceptible to stress.