We describe a method to identify neuropeptides in specific cell-types by the conditional disruption of the gene for carboxypeptidase E (Cpe), an essential neuropeptide-processing enzyme. The loss of Cpe led to the accumulation of neuropeptides containing C-terminal basic residues which are used as tags for affinity-purification followed by identification using quantitative peptidomics. This approach enables the identification of specific forms of neuropeptides in genetically defined populations of cells.