In the present study, the mechanism involved in the enhancement of adriamycin (ADR) accumulation by cepharanthin (CEP) and hyperthermia were examined. The accumulation of ADR and Fluo3 (an indicator of Ca2+) was increased by treatment with CEP. This suggests that ADR accumulation may increase due to increased influx of Ca2+. The ADR accumulation increased with increasing the KCl concentration in the presence of CEP when the KCl concentration was over 200 mM. This demonstrates that ADR accumulation increases with treatment with CEP and increasing the KCl concentration after the cell membrane potential reaches 0. ADR accumulation did not change with the extracellular pH in the absence of NaHCO3. CEP increased the ADR accumulation with increasing extracellular pH, regardless of NaHCO3. This suggests that CEP may affect the H+ flux, and consequently increase ADR accumulation regardless of NaHCO3. Further, it increased with increasing extracellular pH in the presence of NaHCO3, regardless of CEP. It is thought that the interaction between H+ and HCO3- may cause increased ADR accumulation. ADR accumulation was decreased by DIDS and amiloride at 37 degrees C. ADR accumulation in the presence of amiloride at 45 degrees C was increased compared with that at 37 degrees C. However, the accumulation combined with amiloride and hyperthermia was lower than with treatment with ADR only at 37 degrees C. This indicates that ADR accumulation is decreased by amiloride and increased by hyperthermia. ADR accumulation is further decreased by DIDS at 45 degrees C compared with 37 degrees C. This suggests that the H+ flux in hyperthermia is influenced by Cl-/HCO3- exchanger but not Na+/H+ exchanger.