Pharmacological studies have shown that there are multiple serotonin receptor subtypes, which are classified in at least seven classes. . There has been a lack of subtype selective serotonin receptor radioligands suitable for PET imaging of the human brain. The 5-HT1a receptor is of particular interest as it may be involved in the pathophysiology of several neuropsychiatric disorders, like anxiety, depression and schizophrenia. Recently, a potent and selective 5-HT1a receptor antagonist, WAY-100635 (N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridyl)cyclohexanecarboxamide) was developed. Labelling of WAY-100635 in the methoxy position with 11C [2–3] provided the first radioligand for the delineation of 5-HT1a receptors in the human brain using positron emission tomography (PET) . The descyclohexanecarbonyl analogue ([11C]WAY- 100634) was shown to be a labelled lipophilic metabolite in primates with high affinity to 5-HTlA receptors and high ability to enter the brain, so hampering the quantitation of the uptake of the radioligand in vivo . WAY-100635 has recently been labelled with 11C also in the carbonyl position and examined in the human brain with PET .