The aim of this work was to investigate the distribution and developmental changes of Neuropeptide-Y(NPY)and its Y1 receptor(NPYY1)-like immunoreactivity cells in the duck thymus using immunohistochemistry associated with morphological analysis.Studies were carried out on Tianfu ducks on days 14,18,22 and 26 days of embryogenesis(E14,E18,E22 and E26)as well as at 0(neonatal stage),3,5,8,11,20 and 32 weeks of postnatal development(P0,P3,P5,P8,P11,P20 and P32).NPY-like immunoreactivity(NPY-ir)was mainly detected in the epithelial reticular cells and vascular smooth muscles,slightly in the lymphocytes from E26 onwards.On E26,NPY-ir was restricted to the epithelial reticular cells of diffuse forms of Hassall's corpuscle(DHC).The integral optical density(IOD)values of NPY-ir cells in the cortex and medulla did not significantly changed from P0 to 11,and then dramatically rose from P20 to 32.NPYY1-like immunoreactivity(NPYY1-ir)was mainly observed in the lymphocytes,slightly in the epithelial reticular cells of DHC and the vascular smooth muscles from E18 onwards.The IOD values of NPYY1-ir cells significantly increased from E18 to E22,then kept unchanged until P0,and greatly increased on P3,and then remained stable until P11,but dramatically increased from P20 to 32.These results suggest that the possible interaction between NPY and NPYY1 may take place within the duck thymus chiefly during the post embryonic development,which might be of great importance for the function of thymocytes,as well as duck thymus involution.