Human lymphocyte melatonin, through membrane and nuclear receptors binding, acts as an activator in IL-2 production. Antagonism of membrane melatonin receptors using luzindole exacerbates the drop of the IL-2 production induced by PGE(2) in peripheral blood mononuclear and Jurkat cells. This paper studies the melatonin membrane and nuclear receptors interplay in PGE(2)-diminished IL-2 production. The decrease in IL-2 production after PGE(2) and/or luzindole administration correlated with downregulation in the nuclear receptor RORalpha. We also highlighted a role of cAMP in the pathway, because forskolin mimicked the effects of luzindole and/or PGE(2) in the RORalpha expression. Finally, a significant RORalpha downregulation was observed in T cells permanently transfected with inducible MT(1) antisense. In conclusion, we show a novel connection between melatonin membrane receptor signalling and RORalpha expression, opening a new way to understand melatonin regulation in lymphocyte physiology.