Although the glia derived neurotrophic factor (GDNF) is defined as a molecule that maintains neuronal cells, it possesses a range of functions outside the nervous system. For example, it is essential for uretric branching in kidney morphogenesis and for regulating the differentiation of stem cells during spermatogenesis, cardiac, hair follicle and vascular differentiation and the maintenance of immune cells. In the present work, the presence of GDNF in carp peripheral blood leukocytes (PBL) and head kidney cells (HK) was evidenced and its evolutionary importance in both neural and immune systems development was suggested. Using the northern-blot technique, we could observe the expression of two different transcripts of this gene. GDNF upregulation was detected using semi-quantitative PCR, following ex vivo treatment of PBL and HK cells with the immunomodulator AS101 which was previously shown to inhibit IL-10 and to up-regulate GDNF protein levels in human SVG astrocyte cell line, in 6-OHDA hemi-parkinsonian mice in vivo and in rat glomerular mesengial cells in vitro.