RNA interference (RNAi) has considerable potential as a therapeutic strategy, but the development of efficient in vivo RNA delivery methods remains challenging. To this end, we designed and synthesized chemically modified interfering nanoparticles (iNOPs) composed of functionalized poly-l-lysine dendrimers modified with reducible spacers to facilitate release of small interfering RNAs (siRNAs) in vivo. We show that the novel siRNA-iNOP complexes mediate efficient gene-specific RNAi in cultured cells and in mice, where they display enhanced tissue-targeting capabilities. At a clinically feasible dose of 1 mg kg-1, apolipoprotein B (apoB) siRNA-iNOP complexes achieved approximately 40-45% reduction of liver apoB mRNA and plasma apoB protein levels within 48 h of administration to mice, without apparent toxicity. Collectively, these findings demonstrate that siRNA delivery by the modified reducible iNOPs can provide a clinically significant and potentially tissue-specific new approach for RNAi therapy.