The potential of chitosan as a polycationic gene carrier for oral administration has been explored since 1990s. Chitosan has been shown to effectively bind DNA in saline or acetic acid solution and protect DNA from nuclease degradation. In this study, pDNA (plasmid DNA) was encapsulated in chitosan microparticles. Chitosan-DNA microparticles were prepared using a complex coacervation process and stability of plasmid DNA was investigated in this complex. The chitosan-DNA microparticles could protect the encapsulated plasmid DNA from nuclease degradation. Release of pDNA from microparticles was studied in simulated gastric, simulated intestinal medium and acidic PBS (phosphate buffer saline) (pH 4.5) buffer at 37 degrees C, and released pDNA was assayed spectrophotometrically. In vitro release of pDNA from chitosan microparticles was dependent on pH, as the pH of the release medium increased release profile decreased. In in vivo-animal studies blue color was observed with X-gal (4-chloro-5-bromo-3-indolyl-beta-galactosidase) staining of histological stomach and small intestine sections after oral administration of pDNA-chitosan microparticles as an indicator of exogeneous gene expression.