HepG2 cells were pre-incubated with 10 μM berberine at 37°C for 2 h, then the cells were incubated with drug-free HBSS or HBSS containing CCCP (0.25 μM), or verapamil (100 μM), or CCCP (0.25 μM) plus verapamil (100 μM) at 37°C for 0.5, 1, 2, 4 h, respectively. After incubation, the cells were washed three times with chilled PBS and harvested by trypsinisation. The cells were stored in 0.5 ml chilled PBS and the accumulated berberine in the cells was quantified by flow cytometry analysis with excitation at 488 nm and emission at 530 nm. The amount of berberine was normalised to that at zero time point. * indicates P<0.05 while ** indicates P<0.01 versus verapamil treated groups.