This study has been conducted in order to determine whether the CIDR application on Kivircik and Karacabey Merino ewes used for supplying estrous synchronization a stress factor or not. For this purpose; T3 (triiodothyronine), T4 (thyroxine) and cortisol hormone levels of sheep in their blood samples were measured on day 1 when CIDR apparatus were mounted, on day 7 and on day 14 when they were removed. In this study, 80 head of ewes in total were used which consisted of 40 head Kivircik and 40 head Karacabey Merino ewes. 10 ml of blood from jugular veins of sheep were taken at the specified periods into EDTA tubes, and their T3, T4 and cortisol concentrations were identified by commercial RIA kits. In Kivircik race, these values were identified as 1.76 (ng/mL), 7.92 (mu g/dL), 5.30 (mu g/dL) respectively while in Karacabey Merino race as 1.69 (ng/mL), 7.00 (mu g/dL) and 4.19 (mu g/dL) respectively. In terms of T4 and cortisol, the difference between races were found significant (P<0.01) while difference between the periods were found significant only for T3 hormone (P<0.01).