Context: Cationic lipoplexes are less toxic than viral gene vectors and more convenient to prepare but their efficiencies of gene delivery are generally lower. Objective: To develop ortho ester-based, pH-sensitive lipoplexes for efficient gene delivery both in cultured cells and in vivo. Materials and methods: A novel cationic and acid-labile lipid (DOC) containing a cationic headgroup and a cholesterol-derived lipid tail joined together by an acid-labile ortho ester linker was designed and synthesized. DOC was formulated into liposomes with the conical helper lipid DOPE, and then into lipoplexes with plasmid DNA encoding a luciferase reporter gene. The physicochemical properties of the lipoplexes (size, surface charge and pH-sensitivity) were characterized. Gene delivery by DOC/DOPE/DNA lipoplexes was also evaluated in CV-1 cells and in CD-i mice following intratracheal injection. Lipoplexes consisting of the acid-stable cationic lipid DC-Chol were characterized as a control. Results: DOC formed cationic lipoplexes with DOPE and DNA. After incubation at acidic pH 4.6, DOC/DOPE/DNA lipoplexes lost their positive charges and aggregated with one another as a result of DOC hydrolysis. Both in CV-1 cell culture and in CD-1 mice, DOC/DOPE/DNA lipoplexes increased the luciferase gene expression by 5- to 10-fold compared with the analogous but acid-stable DC-Chol/DOPE/DNA lipoplexes. Discussion and conclusion: Incorporation of an acid-labile ortho ester linker into a cationic lipid is a viable approach to enhance gene delivery by the corresponding lipoplexes both in cultured cells and in vivo