In media containing simple nitrogen sources, the highest yields of aminopeptidase were obtained fromAeromonas proteolyticaMerkelet al. when the organism was grown in asparagine or sodium nitrate supplemented with glycerol; growth in a medium containing enzymatically hydrolyzed casein, however, produced 10 times more enzyme than that observed with the best simple nitrogen sources. The presence of glycerol (0.4%) in a 0.2% asparagine medium resulted in 2.5 times more enzyme although growth was not similarly stimulated. When acid-hydrolyzed casein was the nitrogen source, aminopeptidase synthesis was about three times greater than that obtained with asparagine and glycerol but only one-third of that observed when enzymatically hydrolyzed casein was used. The production of aminopeptidase was greatly enhanced by the addition of small quantities of peptide mixtures to media containing either acid-hydrolyzed casein or asparagine as the primary nutrient source. The data suggest that the synthesis of aminopeptidase is under multiple controls which involve the action of short chain compounds and peptides.