Nitric oxide (NO) is a regulator of many biological functions including T helper 1 (Th1)/T helper 2 cells balance. It has been demonstrated that NO inhibits the secretion of interleukin-2 (IL-2) and interferon-gamma on Th1 cells. Here we showed that, in addition to the suppression of IL-2 production, NO-generating agents sodium nitroprusside (SNP) and S-nitroso-N-acetylpenicillamine (SNAP) increased the secretion of IL-4 both in Th2 clones and EL4 T cells. The additive effect was dependent on the dose of SNP and SNAP. Augmentation of IL-4 production was detected with 1 microM SNP, and up to threefold increase in IL-4 secretion could be observed with higher concentrations of SNP/SNAP. NO also weakly increased the activation of IL-4 promoter. In contrast, NO markedly inhibited the induction of IL-2 promoter, which could account for most of the reduction in IL-2 production. Analysis of the transcriptional elements on IL-2 and IL-4 promoters revealed a selective inactivation of NF-kappa B and NF-AT. It is suggested that despite the complex feedback network regulating NO production, the enhanced IL-4 expression would lead to the expansion of Th2 cells once NO is generated.