The expression of the three key peptide processing enzyme families, represented by CPE, PAM, and PC1/3 plus PC2, were examined in MCF-7 and ZR-75-1 breast cancer cell lines. Both of these cell lines express vasopressin receptors as well as the vasopressin gene, but the processing of vasopressin gene-related proteins appears to be limited. Products of the expected size for, CPE, PAM and PC1/PC3 could be amplified by reverse transcription-polymerase chain reaction (RT-PCR) from both cell lines. Cloning and sequencing of these RT-PCR products revealed that each enzyme mRNA had a structure identical to that published for the human form of the respective enzyme. Western analysis provided evidence that mRNAs for these enzymes are translated into proteins. Alternatively, PC2 mRNA was identified to be present in MCF-7 cells both by RT-PCR and Western blot analysis, but could not be demonstrated for ZR-75-1 cells. Our findings suggest that the key processing enzymes needed to generate active vasopressin and other neuropeptide growth factors are present in breast cancer cells.