In order to understand the molecular basis of the synergistic action of interferon gamma (IFN-gamma) and tumour necrosis factor alpha (TNF-alpha) on rat oligodendrocyte development, we studied some aspects of the signalling pathways involved in the regulation of the major histocompatibility complex (MHC) class I and the interferon regulatory factor 1 (IRF-1) gene expression. Two well-defined inducible enhancers of the MHC class I gene promoter, the MHC class I regulatory element (MHC-CRE) and the interferon consensus sequence (ICS), were analysed. Neither IFN-gamma nor TNF-alpha was capable of inducing MHC-CRE binding activity when administrated alone. Following the exposure of oligodendrocytes to IFN-gamma, TNF-R1 expression was transcriptionally induced by the binding of signal transducer and activator of transcription (STAT-1) homodimers to the IFN-gamma activated site (GAS) present in the gene promoter. The upregulation of TNF-R1 allowed TNF-alpha to induce the binding of nuclear factor-kappaB (NF-kappaB) to the MHC-CRE site. With respect to ICS element, IFN-gamma induced IRF-1 binding, that was further enhanced upon co-treatment with TNF-alpha. The existence of a synergism between IFN-gamma and TNF-alpha in stimulating IRF-1 expression at the transcriptional level was supported by IRF-1 promoter analysis: IFN-gamma directly induced the binding of STAT-1 homodimers to the GAS element, while NF-kappaB binding to the kappaB sequence was activated by TNF-alpha only after IFN-gamma treatment. This transcriptional regulation of IRF-1 gene by IFN-gamma and TNF-alpha was confirmed at the mRNA level. The synergism demonstrated in the present study highlights the importance of cytokine interactions in magnifying their biological effects during brain injury and inflammation.