We examined the action mechanisms of enhancers that improve paracellular drug transport. For sodium caprate (C10), the increase in the intracellular calcium level was considered to induce the contraction of calmodulin-dependent actin filaments, followed by dilation of the paracellular pathway. Although decanoylcarnitine (DC) also increased the intracellular calcium level, the action was independent of calmodulin and thus, the action mechanism of acylcarnitines was considered to differ from that of C10. Other acylcarnitines, lauroylcarnitine (LC) and palmitoylcarnitine (PC) and organic acids, tartaric acid (TA) and citric acid (CA) decreased the intracellular ATP level and the intracellular pH. From these results, it was considered that one of the action mechanism of acylcarnitines and organic acids is that the intracellular acidosis increases the calcium level through the decrease in ATP levels, followed by opening the tight junction. Membrane dysfunction which was expected from the above mechanism was assessed by the transport function of electrolytes. Membrane conductance, which was increased by C10, LC and PC, returned to the control value during a 3- to 6-h recovery period. On the other hand, Cl(-) ion secretion, which was obtained from short-circuit current (I(sc)), was decreased by these enhancers, but was normalized by C10 but not by LC and PC. Accordingly, C10 can be considered a safer enhancer than acylcarnitines.