Brain natriuretic peptide (BNP) from 3 different species was cleaved by neutral endopeptidase (NEP) and the products separated by HPLC. The newly formed products were identified by fast atom bombardment or nebulizer-assisted electrospray mass spectrometry to elucidate the sites of proteolysis. Porcine BNP was cleaved at the Arg 8-Leu 9 and Ser 14-Leu 15 bonds. Rat BNP was cleaved at the Arg 23-Leu 24 and Arg 30-Leu 31 bonds. Human BNP was cleaved at the Pro 2-Lys 3, Met 4-Val 5 and Arg 17-Leu 18 bonds. The Cys-Phe bond which is present in all species of BNP is not cleaved by NEP.