Instability and inadequate biodistribution of double-stranded RNA are major drawbacks to the clinical use of RNA interference. This work compares chemical modification and nanoparticle formulation as strategies to improve the systemic delivery of small interfering RNA (siRNA). Variable levels of chemical modified siRNA, either naked or within nanoparticle, were intravenously injected into mice to study temporal stability and biodistribution detected by direct radioactive labeling or by northern blotting. Naked siRNA showed rapid renal clearance, with circulatory half-life of <5 minutes that could be extended to >30 minutes by cholesterol conjugation. The integrity of the chemically stabilized siRNA was maintained in blood for at least 30 minutes, whereas, unmodified siRNA duplex was degraded within 1 minute. Intact chemically modified siRNA could also be detected in all analyzed organs at 30 minutes but disappeared at 24 hours, except for heavy locked nucleic acid (LNA)-modified and cholesterol-conjugated siRNA in the lungs. Chitosan, liposomal, or JetPEI formulation greatly improved the stability and biodistribution of siRNA. Interestingly, high siRNA accumulation of the chitosan/siRNA formulation within the kidney was observed 24 hours postadministration. This comparative study highlights improvements to siRNA stability and pharmacokinetics, key determinants for development of clinically relevant RNAi therapeutics.