In this open, 30-day trial, the pharmacokinetics, safety and tolerability of a combination therapy of risperidone (4 or 6 mg/day)and fluoxetine (20mg/day from day 6) were evaluated in 11 psychotic inpatients. CYP2D6 genotyping revealed that 3 and 8 patients were poor metabolizers (PMs) and extensive metabolizers (EMs) of debrisoquine, respectively. The mean (+/- SD) AUC of risperidone increased from 83.1 +/- 46.8 ng.h/ml and 398.3 +/- 33.2 ng.h/ml (monotherapy) to 345.1 +/- 158.0 ng.h/ml (p < 0.05) and 514.0 +/- 144.2 ng.h/ml (p < 0.001) when coadministered with fluoxetine in EMs and PMs, respectively. The AUC of the active moiety (risperidone plus 9-hydroxy-risperidone) increased from 470.0 +/- 170.0 ng.h/ml to 663.0 +/- 243.3 ng.h/ml (p < 0.05)and from 576.3 +/- 19.6 ng.h/ml to 788.0 +/- 89.1 ng.h/ml (ns) in EMs and PMs, respectively. In EMs, the AUC of 9-hydroxy-risperidone remained similar (monotherapy vs. combination therapy: 386.8 +/- 153.0 ng.h/ml vs. 317.7 +/- 125.2 ng.h/ml, ns),whereas it increased in PMs (178.3 +/- 23.5 ng.h/ml vs. 274.0 +/- 55.1 ng.h/ml (p < 0.05)). Ten of the 11 patients showed a clinical improvement (reduction of 20% or more in total PANSS score and 70% on the mean MADRS score compared to baseline). The severity and incidence of extrapyramidal symptoms and adverse events did not significantly increase when fluoxetine was added.