Extension of in vivo nucleic acid transfection techniques and increased information about those transfection properties and side effects are urgently needed to advance biological research and drug therapy. Tissue pressure-mediated transfection, involving lightly pressing the target tissue after intravenous injection of plasmid DNA or small-interfering RNA (siRNA), is a promising approach because of its high transfection efficiency and resulting low tissue damage. In this study, the gene expression/silencing properties and proinflammatory cytokine production associated with tissue pressure-mediated transfection were evaluated to extend its application. We have found that tissue pressure-mediated transfection can be applied to plasmid DNA and siRNA transfection to the spleen and siRNA transfection to the liver. In addition, we have demonstrated that these methods induce little production of proinflammatory cytokines, including tumor necrosis factor-alpha, interleukin (IL)-6, IL-12, and interferon-gamma. Moreover, we succeeded in controlling and quantifying the degree of pressure on the spleen and kidney and found that 0.59 N/cm(2) is sufficient for efficient and highly reproducible plasmid DNA transfection to the spleen and kidney in mice. Tissue pressure-mediated transfection of the kidney, liver, and spleen exhibits well-balanced characteristics including (1) simple and convenient manipulation, (2) tissue-specific, effective broad transfection properties, and (3) a low inflammatory response. Therefore, this information could be useful for a molecular-level mechanism analysis of diseases at an individual level in mammals, exploration of therapeutic target molecules and evaluation of gene therapy and nucleic acid-based therapy approaches, as well as potential clinical applications.