Many functional studies show that melatonin plays a fundamental role in neuroimmunomodulation. In this paper, we have extended our studies on the influence of melatonin on IL-2 and IL-6 production by human peripheral blood mononuclear cells (PBMCs) by comparing the effects of the specific membrane receptor agonist S 20098, the RZR/ROR(alpha) receptor agonist CGP 52608, and structurally related thiazolidinediones. Melatonin bound to membranes as well as to nuclei of human PBMCs with about the same affinity (IC50 values around 5 nM). S 20098 bound to PBMC membranes but not to PBMC nuclei, although the affinity was at least 100 times lower than that of melatonin; this compound did not stimulate cytokine production. In contrast, all four CGP compounds did not bind to PBMC membranes, while binding to nuclei exhibited IC50 values comparable to those of melatonin. The thiazolidinediones activating the RZR/ROR(alpha) receptor (CGP 52608, CGP 53079) also increased IL-2 and IL-6 production. CGP 55644 had no effect on cytokine production and antagonized the effects of CGP 52608 on IL-2 and IL-6 production; moreover, CGP 55644 decreased the enhanced IL-2 production caused by melatonin. Results obtained in monocyte cultures resembled closely those shown in PBMCs. The results reported in this paper confirm the involvement of a nuclear mechanism in the melatonin effects on cytokine production in human PBMCs. We have also shown a synergistic effect of S 20098 and CGP 52608, suggesting a possible link between nuclear and membrane melatonin receptors in PBMCs.