Substance P (SP) has been reported to induce inflammatory cytokine production in human neuroglial cells and peripheral lymphoid cells as well. In order to evaluate the potency of novel non-peptide antagonists of the tachykinin receptors as inhibitors of SP-induced cytokines, we used the astrocytoma cell line U373MG and blood mononuclear cells as models of central and peripheral SP-target cells, respectively. In the first part of this study, we showed that SR 140333, an NK1 tachykinin receptor antagonist, was able to inhibit strongly the SP-induced production of interleukin (IL)-6 and IL-8 in the astrocytoma cell line. The antagonistic activity of SR 140333 toward SP-induced cytokine production was specific and could not be attributed to a general anti-cytokine effect, since cytokine release induced by another inflammatory protein such as IL-1beta was not blocked by this compound. In addition, NK2 and NK3 agonist neuropeptides were at least 1000-fold less effective than SP, while SR 48968 and SR 142801 which are selective NK2 and NK3 receptor antagonists, respectively, displayed a 2.5-3 orders of magnitude lower inhibitory potency than SR 140333. All these data indicated that SR 140333 blocked SP-induced cytokine production in U373MG astrocytic cells via a specific NK1 receptor-mediated process. Since SP has also been described to trigger peripheral blood mononuclear cells (PBMNC) or monocytes to release inflammatory cytokines, we attempted, in the second part of this study, to evaluate the potential antagonistic effect of our compounds on these cells. Experiments on human PBMNC from different donors were carried out to determine first their pattern of cytokine production upon SP stimulation. Surprisingly, we noticed that SP at concentrations ranging from 0.1 to 1000 nM was unable to stimulate the release of any inflammatory cytokine tested. This raises the question of the specificity of the reported in vitro effects of SP on cytokine production by human peripheral immune cells.