With the aim of imaging and quantification of oxytocin receptors (OTRs) in living brain using positron emission tomography (PET), we developed a 18F-labeled small molecule radiotracer and investigated its in vivo pharmacokinetics in mice and pig. [18F]6b (KD = 12.3 nM) was radiolabeled by a two-step procedure using a microwave system with radiochemical yields of 26.9 ± 4.7%. Both organ distribution and small animal PET studies revealed limited brain uptake of [18F]6b in mouse (mean SUV of 0.04 at 30 min p.i.). Besides, significant radioactivity uptake in the pituitary gland was observed (SUV of 0.7 at 30 min p.i.). In a dynamic PET study in one piglet, we detected a higher uptake of [18F]6b in the olfactory bulb (SUV of 0.34 at 30 min p.i.) accompanied by a low uptake in the whole brain. In vitro autoradiographic studies on porcine brain sections indicated interaction of [18F]6b with several off-target receptors.