Interleukin 1β (IL-1β) induces expression of the inducible nitric- oxide synthase (iNOS) with concomitant release of nitric oxide (NO) from glomerular mesangial cells. These events are preceded by activation of the c- Jun NH2-terminal kinase/stress-activated protein kinase (JNK/SAPK) and p38(MAPK). Our current study demonstrates that overexpression of the dominant negative form of JNK1 or p54 SAPKβ/JNK2 significantly reduces the iNOS protein expression and NO production induced by IL-1β. Similarly, overexpression of the kinase-dead mutant form of p38α(MAPK) also inhibits IL-1β-induced iNOS expression and NO production. In previous studies we demonstrated that IL-1β can activate MKK4/SEK1, MKK3, and MKK6 in renal mesangial cells; therefore, we examined the role of these MAPK kinases in the modulation of iNOS induced by IL-1β. Overexpression of the dominant negative form of MKK4/SEK1 decreases IL-1β-induced iNOS expression and NO production with inhibition of both SAPK/JNK and p38(MAPK) phosphorylation. Overexpression of the kinase-dead mutant form of MKK3 or MKK6 demonstrated that either of these two mutant kinase inhibited IL-1β-induced p38(MAPK) (but not JNK/SAPK) phosphorylation and iNOS expression. Interestingly overexpression of wild type MKK3/6 was associated with phosphorylation of p38(MAPK); however, in the absence of IL-1β, iNOS expression was not enhanced. This study suggests that the activation of both SAPK/JNK and p38α(MAPK) signaling cascades are necessary for the IL-1β-induced expression of iNOS and production of NO in renal mesangial cells.