Palmitoyl carnitine chloride (PCC) has been shown to be an effective enhancer of intestinal transport of hydrophilic molecules. The exact mechanism by which the epithelial barrier function is decreased is not clear. In an attempt to elucidate the mechanism of action of PCC, we studied the relationship among absorption enhancement, cell viability and tight junction protein localization in the human colonic Caco-2 cell line and the rat small intestinal cell line IEC-18. Filter-grown cells were exposed to 0 to 1 mM PCC for 30 min, and the efficacy of PCC treatment was determined by assessing the transepithelial electrical resistance and the apparent permeability for mannitol and PEG-4000. Membrane lysis and cytotoxicity were assessed by measurement of lactate dehydrogenase leakage and uptake of propidium iodide and neutral red. The immunolocalization of the tight junctional protein ZO-1 was quantified using CSLM and image-processing software. In both cell lines, PCC caused a dose-dependent decrease in transepithelial electrical resistance and a concomitant increase in the permeability for mannitol and PEG-4000. The transport enhancement was accompanied by an increase in apical membrane permeability and a reduction in cell viability. At higher PCC concentrations (≥ 0.4 mM), the distribution of the tight junctional protein ZO-1 was changed and cells were unable to recover viability. PCC is effective as an absorption enhancer for hydrophilic macromolecules. However, lytic effects on the cell membrane and reduced cell viability were concomitant with transport enhancement.