Drugs acting on the 5-HT 1A receptor are used in the treatment of depression, generalized anxiety disorder, and schizophrenia. This study investigated 5-HT 1A receptor occupancy by the 5-HT 1A agonist drugs flesinoxan (a highly selective probe for the 5-HT 1A receptor) and ziprasidone (a novel atypical antipsychotic drug). Using a within-subject design, 14 healthy volunteers each received two positron emission tomography scans using the selective 5-HT 1A antagonist radiotracer [ 11 C]WAY-100635. One scan constituted a baseline, while the other followed either 1 mg flesinoxan or 40 mg ziprasidone orally. In addition, rats were pretreated with intravenous flesinoxan at doses ranging from 0.001 to 5 mg/kg then [ 11 C]WAY-100635 binding measured ex vivo. Cerebral cortical and hippocampal regions of interest, and cerebellar reference regions were sampled to estimate 5-HT 1A receptor occupancy (inferred from reductions in specific radioligand binding). In man, occupancy was not significant despite volunteers experiencing side effects consistent with central serotonergic activity. The mean cerebral cortex occupancy (71 SD) for flesinoxan was 8.7% (713%), and for ziprasidone 4.6% (717%). However, in rats, flesinoxan achieved significant and dose-related occupancy (17-57%) at 0.25 mg/kg and above. We conclude that 5-HT 1A receptor agonists produce detectable occupancy only at higher doses that would produce unacceptable levels of side effects in man, although lower doses are sufficient to produce pharmacological effects. The development of agonist radiotracers may increase the sensitivity of detecting agonist binding, as 5-HT 1A antagonists bind equally to low-and high-affinity receptor states, while agonists bind preferentially to the high-affinity state.