Invariant natural killer T (iNKT) cells are CD1d-restricted, glycolipid-reactive lymphocytes with potent immunoregulatory characteristics. Although recent years have witnessed intensified interest in iNKT cells, little is known about intracellular signaling pathways that control iNKT cell responses, including those mediated by mitogen-activated protein kinases (MAPKs). We employed selective inhibitors of ERK1/2, JNK and p38 to examine the importance of these MAPKs in iNKT cell responses to the prototype glycolipid antigen alpha-galactosylceramide (alpha GC). Activation of DN32.D3 iNKT cells in the presence of PD98059 led to decreased interleukin (IL)-2 production, indicating a role for ERK in mouse iNKT cell responses. In contrast, addition of the JNK inhibitor SP600125 to cultures did not significantly affect cytokine production, suggesting that JNK is not critically needed for iNKT cell responses. Interestingly, selective inhibition of p38 by either SB203580 or SK&F 86002 resulted in augmented IL-2 production by DN32.D3 cells after stimulation with alpha GC. This was also evident when iNKT cells were stimulated with an anti-CD3 monoclonal antibody thus bypassing the requirement for CD1d-mediated antigen presentation, indicating that p38 inhibition affects signal transduction downstream of iNKT cells' T cell receptors. Primary splenic iNKT cells similarly exhibited enhanced cytokine response to alpha GC when cultured in the presence of p38 inhibitors. Importantly, in vivo administration of SB203580 resulted in higher IL-4 and interferon-gamma secretion in alpha GC-treated mice. These results demonstrate that MAPKs play distinct signaling roles in iNKT cells and that both in vitro and in vivo iNKT cell responses to glycolipid antigens can be negatively modulated by p38.