The in vivo and ex vivo distributions and the pharmacological profile of the fluorinated phenylpiperazine derivative p-[ 18 F]MPPF (4-(2Ј-methoxyphenyl)-1-[2Ј-(N-2Љ-pyridinyl)-p-fluorobenzamido]-ethylpiperazine) were evaluated in the cat brain as a potential selective antagonist for 5-HT 1A receptors using PET. After intravenous injection of p-[ 18 F]MPPF in cats, there was a rapid accumulation of radioactivity in the brain, with 4% of the total radioactivity injected present in the brain at 4 minutes postinjection. The highest uptakes of radioactivity were observed in the hippocampus and cingulate cortex, regions known to be rich in 5-HT 1A receptors, whereas lower levels of radioactivity were observed in the cerebellum. The mean ratio of radioactivity in the hippocampus to the cerebellum was 4.29 (SD ϭ 0.21; n ϭ 5) from 40 to 90 minutes postinjection of p-[ 18 F]MPPF. The corresponding ratio for the cingulate cortex was 3.01 (SD ϭ 0.16; n ϭ 5). Specific binding in the hippocampus and the cingulate cortex was markedly reduced following injection of unlabeled WAY-100635 and pindolol but was unaffected by treatment with ␣1, 5-HT 2 , or reuptake inhibitor agents indicating reversibility and selectivity of p-[ 18 F]MPPF binding to 5-HT 1A receptors. Ex vivo autoradiographic study with p-[ 18 F]MPPF in cat brain sections showed labeling of areas rich in 5-HT 1A receptors with a regional brain distribution that closely matched that observed using PET. These results indicate that p-[ 18 F]MPPF may be a useful candidate for noninvasive PET imaging of 5-HT 1A receptors in the living human brain.