In an effort to develop a peptide-based radiopharmaceutical for the detection of tumors overexpressed vasoactive intestinal peptide receptors with positron emission tomography, we have prepared a novel [R 8,15,21 ]-VIP from non-target tissues and specific uptakes by tumors realized higher tumor-to-muscle ratio (3.55) and tumor-to-blood ratio (2.37) 60 min postinjection. Clear difference was observed between the blocking and unblocking experiments in biodistribution and whole body radioautography. [ Vasoactive intestinal peptide (VIP, Scheme 1A), with a wide range of biologic activities, was extracted from porcine duodenum previously by Said and Mutt (1). It was found that biologic effects of VIP are mediated through interactions with a large family of G-protein-coupled receptors in cell membrane. Two receptor subclasses named VPAC1 and VPAC2, are expressed in a number of normal tissues, such as lung, liver, gastrointestinal tract, and peripheral blood vessels (2-7), and that many tumor cells, such as neuroendocrine tumors, brain tumors, and adenocarcinomas of colon, pancreas, stomach, and liver, contain high-density VIP receptors (8). Therefore, radiolabeled VIP is a preferable agent for imaging gastrointestinal and neuroendocrine tumors (9).Since 1994, however, the argument about detection efficiency of this compound remained unclarified, which could be related to fast proteolytic degradation of VIP in vivo and, as a result, the radioactivity uptakes in tumor might be too low to distinguish small tumors, even to cause false-negative results (7,(10)(11)(12). A VIP analog with longer half-life of proteolytic degradation and higher receptor-binding affinities, nevertheless, would need chemical modifications of the VIP. Therefore, 99m Tc-labeled structure-modified VIP analogs of VIP-AbaGly-Gly-(D)-Ala-Gly (TP3654) and [des-Met17,S-(CH 2 CO-Gly-Gly-CysLys-amide) Hcy17] VIP (P1666), with longer half-life of proteolytic degradation and high receptor-binding affinities, are promising agents for localizing tumor-expressing VIP receptors, and are used for single photon emission computed tomography imaging (SPECT; 13-15).Positron emission tomography (PET), an imaging technique with 18 F or other positron emission radionuclides, is more powerful than computed tomography (CT), magnetic resonance imaging (MRI) and SPECT for early detection of carcinomas, hence higher cure rate than tumors diagnosed in advanced stage (16,17). Therefore, labeling VIP with 18