The objectives of this study were (a) to compare the aminopeptidase activity in the Peyer's patches of the jejunum and ileum of the albino rabbit against that in the adjacent patch-free segments and (b) to determine the relative sensitivities of the aminopeptidase activity in the Peyer's and non-Peyer's patches to aminopeptidase inhibitors and penetration enhancers. The results indicated that the Peyer's patches were about equal in aminopeptidase activity in the jejunum and in the ileum but were only 20-30% as rich in aminopeptidase activity as their neighboring patch-free areas. Compared to non-Peyer's patches, the aminopeptidase activity in the Peyer's patches was not as sensitive to the inhibitory effect of amastatin. It was, however, much more sensitive to the inhibitory effect of puromycin and p-chloromercuribenzoate and was somewhat more sensitive to the inhibitory effect of Na deoxycholate, Na glycocholate, and polyoxyethylene-9-lauryl ether. Therefore, based on substrate preferences and on the relative sensitivity of aminopeptidase activity to inhibition by aminopeptidase inhibitors and penetration enhancers, the relative proportions of various aminopeptidases in the Peyer's patches and in the non-Peyer's patches are likely different.