Abstract— The ability of guinea-pig brain to hydrolyse peptides and aminoacyl amides was investigated. The majority of hydrolase activity against both peptides and aminoacyl amides tested was found to reside in the 30,000 g supernatant. Starch gel electrophoretograms of the 30,000 g supernatants showed that seven of the twelve peptides tested were each hydrolysed by more than one peptide hydrolase and that each of the peptide hydrolases observed were capable of hydrolysing more than one peptide. The peptide hydrolases in the 30,000 g supernatant fraction of guinea-pig brain were found to have very similar electrophoretic mobilities to the peptide hydrolases in the 30,000 g supernatant fraction from guinea-pig intestinal mucosa. Only one hydrolase with activity against L-Leu-NH, could be detected in 30,000 g supernatant of guinea-pig brain using starch gel electrophoresis followed by staining and its electrophoretic mobility was identical to that of one of the peptide hydrolases in the same fraction.