We investigated the regulation of major histocompatibility complex (MHC) class I expression by inflammatory cytokines and nitric oxide (NO) in rat mesangial cells by enzyme-linked immunosorbent assay and flow cytometry. MHC class I molecule expression on mesangial cells was significantly stimulated by interferon gamma, tumor necrosis factor alpha and interleukin 1beta, but not by interleukin 6, in a dose-dependent manner. Addition of SIN-1, an NO donor, did not affect the expression of cytokine-induced MHC class I expression. These results suggest that under inflammatory conditions mesangial cells may act as antigen-presenting cells in response to stimulation by cytokines and may be involved in the pathogenesis of immune-mediated glomerular disease.