PACAP (pituitary adenylyl cyclase-activating peptide) is a peptide that exists in 2 forms, 27 or 38 amino acids, and is related to vasoactive intestinal peptide (VIP). Three related class B GPCRs, PAC1, VPAC1 and VPAC2, bind to PACAP; however, VPAC1 and VPAC2 have a much higher affinity for VIP than does PAC1 (Vaudry et al., 2000). Several splice variants of PAC1 result in proteins that differ at the N-terminus and third intracellular loop; these variants differ in their affinities for PACAP and abilities to activate Gq and Gs. High expression of PAC1 is observed in the CNS and the adrenal medulla. Studies with PAC1-null mice indicate that PAC1 plays important roles in regulation of circadian rhythms, neutrophil migration, and pulmonary vascular tone (Hannibal et al., 2001; Martinez et al., 2005; Otto et al., 2004). PAC1-long membrane preparations are crude membrane preparations made from our proprietary stable recombinant cell lines to ensure high-level of GPCR surface expression; thus, they are ideal HTS tools for screening of antagonists of PAC1-long interactions with PACAP27. The membrane preparations exhibit a Kd of 2.7 nM for [125I]-PACAP27. With 5 mg/well PAC1-long Membrane Prep and 0.75 nM [125I]-PACAP27, a greater than 12-fold signal-to-background ratio was obtained.