This study evaluated in vivo gene deliver y and subsequent gene expression within cells of the synovium in the presence of static and pulsating magnetic field application following intraar ticular injection of super paramagnetic iron oxide nanopar ticles linked to plasmids containing reporter genes encoding for fluorescent proteins. Plasmids encoding genes for either green fluorescent protein or red fluorescent protein were bound to super paramagnetic nanopar ticles coated with polyethyleneimine. Larger (200–250 nm) and smaller (50 nm) nanopar ticles were compared to evaluate the effects of size on transfection efficiency as well as any associated intraar ticular reaction. Comparisons between groups were evaluated at 24, 72, and 120 h time periods. Inflammator y response was mild to moderate for all injected par ticles, but was present in the majority of synovial membrane samples evaluated. Larger par ticles tended to be associated with more inflammation than smaller ones. Never theless, intraar ticular application of both experimental and control nanoparticles were well tolerated clinically. Gene expression as determined by obser vation of either green or red intracellular fluorescence was difficult to assess by both epifluorescent light, and confocal microscopy. An insufficient concentration of nanopar ticles in relation to joint volume likely resulted in a limited number of samples with positive evidence of iron staining and with suspected positive evidence of cells expressing fluorescent proteins. Our results indicate that intraar ticular administration of functionalized super paramagnetic iron oxide nanopar ticles resulted in a mild to moderate synovitis and there was in conclusive evidence of gene expression. Fur ther research is warranted to determine the best and most effective repor ter assay for assessment of the in vivo gene deliver y into the joints. In addition, the best suited concentration and size of nanopar ticles, which will optimize gene delivery and expression, while minimizing intraarticular inflammation, needs to be determined.