The control of neuropeptide function is partially accomplished by aminopeptidases (neuropeptidases), which are the most abundant proteolytic enzymes in brain. Their analysis represents an important and quick tool to reflect the functional status of their endogenous substrates. Here, we describe an improved fluorometric method for the determination of neuropeptidase activities based on the fluorescence produced by β-naphthylamine when released from the artificial substrates aminoacyl-β-naphthylamides (arylamides) under the hydrolytic action of these enzymes.