Duloxetine is an inhibitor of serotonin and norepinephrine uptake, and is being developed as a new antidepressant. In the present study, using healthy volunteers who took 20 mg of duloxetine for 7 days, the plasma concentrations of duloxetine and the ex vivo serotonin uptake rate in the platelets were simultaneously monitored during and after administration. Furthermore, a comparison was made by measuring parameters for serotonin uptake in vitro and [3H]paroxetine binding before and after administration. Actual values of the uptake inhibition rate ex vivo were stronger than those expected in spite of the dilution of plasma in the experiment, and the inhibitory effect was seen even after the drug was no longer detected in plasma. No significant changes were observed in Vmax, Km, Bmax or Kd. Thereafter, the effect of the washing procedure was examined in platelets treated with different antidepressants in vitro. The minimum effect was seen in platelets treated by duloxetine or paroxetine, while desipramine-treated platelets showed susceptibility to the procedure. These results suggest that duloxetine was hardly dissociated from the serotonin uptake site, which was responsible for the strong and long-lasting effect of plasma.