Endo-oligopeptidase A (EOPA, formerly EC 3.4.22.19), a thiol-activated oligopeptidase, is able to degrade both bradykinin and neurotensin, and also to convert enkephalin-containing peptides into enkephalins. The expression of this enzyme was studied in the rat brain by in situ hybridization using non-radiotopic probes. The distribution of EOPA transcripts included many regions of the rat central nervous system, with higher expression in some regions, such as the hippocampus, cerebellum, and basal nucleus of Meynert. The marked EOPA expression in these areas could be anticipated, since they are rich in neuropeptides that are known to be EOPA substrates in vitro. The data characterize a widespread occurrence of EOPA in the rat brain and reinforce the suggestion of a critical role for EOPA in peptide processing.