In vitro biotransformation of a newly sequenced neuropeptide of 17 amino acid residues, named nociceptin and orphanin FQ by two separate research groups, was studied in human blood using matrix-assisted laser desorption/ionization mass spectrometry. Processing was carried out in freshly drawn blood incubated at 37 degrees C for various time periods. It was found that cleavage at peptide linkage Phe1-Gly2 was the predominant biotransformation pathway. Nociceptin (2-17) was the major biotransformation product. Further processing also occurred with the formation of a variety of minor biotransformation products. Cleavages at basic amino acid residues were observed, although these were not major biotransformation pathways found under these in vitro experimental conditions. Biotransformation of nociceptin followed a similar pattern to that of another neuropeptide, the endogenous opioid dynorphin A(1-17), but it appeared that nociceptin was more resistant to biotransformation in human blood in vitro than dynorphin A(1-17).