[carbonyl-11C]WAY-100635 is a new radioligand which can be used with positron emission tomography (PET) to provide high contrast delineation of human brain regions that are rich in 5-HT1A receptors. In the present PET study, the binding of [carbonyl-11C]WAY-100635 was characterized in the cynomolgus monkey brain. Pretreatment with each of the two reference compounds, WAY-100635 and 8-OH-DPAT, as well as the drugs buspirone and pindolo, induced a marked inhibition of [carbonyl-11C]WAY-100635 binding in the neocortex and the raphe nuclei. A preliminary Scatchard analysis yielded 5-HT1A receptor density values of the same order as those that have been reported in vitro. The study shows that [carbonyl-11C]WAY-100635 binds specifically to 5-HT1A receptors in the primate brain and has potential for determination of 5-HT1A receptor occupancy and density in psychiatric patients.