Adenosine receptor agonists have anti-inflammatory properties and modulate immune responses partly by inhibiting pro-inflammatory cytokine production by monocytes. We investigated signal transduction mechanisms by which adenosine receptor activation inhibits tumor necrosis factor-alpha (TNF-alpha) production. Phorbol-12-myristate-13-acetate (PMA) and phytohemagglutinin treatment of human pro-monocytic U937 cells increased TNF-alpha protein release. Activation of adenosine receptors up to 1 hr following stimulation with PMA/phytohemagglutinin significantly inhibited TNF-alpha protein release indicating that inhibition of TNF-alpha occurred post-transcriptionally. The adenosine receptor agonist 2-p-(carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride (CGS 21680) decreased stability and half-life of PMA/phytohemagglutinin-induced TNF-alpha mRNA from 80 to 37 min. p38 signaling pathways control TNF-alpha mRNA stability in macrophages and we confirmed in our cells that p38 was involved in controlling TNF-alpha release post-transcriptionally. Activation of adenosine receptors with CGS 21680 decreased phospho-p38 protein levels. These data suggest that adenosine receptor activation regulates TNF-alpha release post-transcriptionally by decreasing mRNA stability through a mechanism involving inhibition of p38 activity.