In in vitro uptake experiments, duloxetine inhibited [3H]5-hydroxytryptamine (5-HT) and [3H]norepinephrine (NE) uptake in hippocampus slices of control rats with IC50 values of 28 and 46 nM, respectively. The uptake of both[3H]5-HT and [3H]NE was equipotently inhibited in hippocampus slices prepared from rats treated for 2 days with different doses of duloxetine (5, 10, 15 and 20 mg/kg/day s.c.). In in vivo electrophysiological experiments in the hippocampus, the effects of duloxetine on the suppression of CA3 pyramidal neuronal firing activity by microiontophoretically applied 5-HT and NE were examined with two modes of administration. Five successive i.v. injections (2 mg/kg each) significantly and dose-dependently prolonged the recovery time of the firing activity of hippocampus CA3 pyramidal neurons from the 5-HT applications. A 2-day treatment (10, 15 and 20 mg/kg/day s.c.) also increased the recovery time in a dose-dependent manner. Whereas the recovery time from NE applications was unaffected by low doses of duloxetine (2 mg/kg i.v.; 10 mg/kg/day for 2 days), it was prolonged significantly by higher doses (8 and 1 0 mg/kg iv.; 20 mg/kg/day for 2 days). Acute i.v. injections of duloxetine suppressed the spontaneous firing activity of dorsal raphe 5-HT and locus ceruleus NE neurons with ED50 values of 99 and 475 microgram/kg, respectively. Taken together, the present results confirmed that duloxetine is a dual 5-HT/NE uptake inhibitor. Furthermore, the results obtained in in vivo experiments indicate that duloxetine has a preferential inhibitory effect on the 5-HT transporter.