Several peptide hormones and neurotransmitters are produced by cleavage at the monobasic processing sites. An endoprotease capable of cleaving a dynorphin peptide at the monobasic processing site is secreted from the rat anterior pituitary lactotrophic cell line, GH4C1. When characterized by fast protein liquid chromatography using an ion exchange column, the majority of the endoprotease activity elutes as a single symmetrical peak around 0.3 M NaCl. The protease inhibitor profile suggests that the activity is due to putative thiol protease. These enzymatic properties are similar to a monobasic processing enzyme previously found in bovine pituitary and in the rat brain. The secretory pathway which contains the enzyme activity in GH4C1 cells was characterized by stimulation of secretion by thyrotropin releasing hormone, forskolin, phorbol ester, or potassium chloride. The secretion of the enzyme activity was substantially increased by these compounds suggesting that the GH4C1 cells secrete the activity via the regulated pathway. A hormonal treatment of the GH4C1 cells which has been previously shown to produce a substantial increase in the number of secretory granules and ir-prolactin has been found in this study to elevate this enzyme activity 2-fold. This increase is similar to that seen in the carboxypeptidase E activity, another putative peptide hormone processing enzyme activity. These data suggest that the peptide processing activity is regulated to a small but significant extent and is coordinately regulated with carboxypeptidase E activity.