The in-vitro potency and selectivity, in-vivo binding affinity and effect of the 5-HT6R antagonist Lu AE58054 ([2-(6-fluoro-1H-indol-3-yl)-ethyl]-[3-(2,2,3,3-tetrafluoropropoxy)-benzyl]-amine) on impaired cognition were evaluated. Lu AE58054 displayed high affinity to the human 5-HT6 receptor (5-HT6R) with a Ki of 0.83 nm. In a 5-HT6 GTPγS efficacy assay Lu AE58054 showed no agonist activity, but demonstrated potent inhibition of 5-HT-mediated activation. Besides medium affinity to adrenergic α1A- and α1B-adrenoreceptors, Lu AE58054 demonstrated >50-fold selectivity for more than 70 targets examined. Orally administered Lu AE58054 potently inhibited striatal in-vivo binding of the 5-HT6 antagonist radioligand [3H]Lu AE60157 ([3H]8-(4-methylpiperazin-1-yl)-3-phenylsulfonylquinoline), with an ED50 of 2.7 mg/kg. Steady-state modelling of an acute pharmacokinetic/5-HT6R occupancy time-course experiment indicated a plasma EC50 value of 20 ng/ml. Administration of Lu AE58054 in a dose range (5–20 mg/kg p.o.) leading to above 65% striatal 5-HT6R binding occupancy in vivo, reversed cognitive impairment in a rat novel object recognition task induced after subchronic treatment for 7 d with phencyclidine (PCP 2 mg/kg b.i.d., i.p. for 7 d, followed by 7 d drug free). The results indicate that Lu AE58054 is a selective antagonist of 5-HT6Rs with good oral bioavailability and robust efficacy in a rat model of cognitive impairment in schizophrenia. Lu AE58054 may be useful for the pharmacotherapy of cognitive dysfunction in disease states such as schizophrenia and Alzheimer's disease.