The rat pancreatic acinar cell line AR 4-2J is endowed with numerous PACAP type I receptors (PACAPR1). The cDNA of this receptor was PCR amplified at low stringency using two degenerate primers based on conserved sequences in the TM2 and TM6 segments of secretin, parathormone and calcitonin receptors. One additional amplified band of 574 bp possessed an original 84 bp insert. This fragment, when used as probe for Northern blot analysis, revealed a high M(r) (about 7.5 kb) transcript in AR 4-2J cells and also in rat brain. Screening a lambda Uni-Zap bacteriophage library of AR 4-2J cell cDNA yielded one hybridizing clone with an ORF of 1254 bp. The translated 418 amino acid peptide showed 206 identities with rat VIP receptors and 176 identities with secretin receptors. This sequence homology, together with the mRNA distribution, and the expression study of a similar cDNA published very recently (8) indicated that we had cloned PACAPR1 except for its 77 N-terminal amino acids. Its putative I3 loop contained a unique additional 28 amino acid sequence (with four hemi-cystines and several serine, threonine and basic residues). Using RT-PCR we then demonstrated the coexistence of a second form of mRNA, without this added insert, in DNAse-pretreated RNAs from both AR 4-2J cells and normal rat brain. This indicated that common alternative splicing provokes the diversification of PACAP type I receptors into PACAPR1A (unspliced) and PACAPR1B (spliced) variants.