Neuropeptide Y (NPY), a classical sympathetic comediator, regulates immunological functions including T cell activation and migration of blood leukocytes. A NPY-mediated neuroimmune cross-talk is well conceivable in sympathetically innervated tissues. In denervated, e.g., transplanted organs, however, leukocyte function is not fundamentally disturbed. Thus, we hypothesized that NPY is expressed by blood leukocytes themselves and regulated during inflammation. NPY mRNA and peptide expression were analyzed in mononuclear leukocytes isolated from the blood vessels of healthy rat kidneys, as well as from the blood vessels of isogeneic and allogeneic renal grafts transplanted in the Dark Agouti to Lewis or in the Fischer 344 to Lewis rat strain combination. Depending on the donor strain, acute allograft rejection is either fatal or reversible but both experimental models are characterized by massive accumulation of intravascular leukocytes. Leukocytes, predominantly monocytes, isolated from the blood vessels of untreated kidneys and isografts expressed high amounts of NPY mRNA and peptide, similar to expression levels in sympathetic ganglia. During acute allograft rejection, leukocytic NPY expression drastically dropped to approximately 1% of control levels in both rat strain combinations. In conclusion, NPY is an abundantly produced and tightly regulated cytokine of mononuclear blood leukocytes.