Several peptidergic PRL-releasing factors (PRFs) have been described; however, none have been proven to be of primary physiological importance in the control of hormone release. Similarly dopamine withdrawal alone cannot completely explain the profiles of PRL secretion observed under a variety of conditions. We describe here the isolation in semipurified form of both a PRF and a PRL-inhibiting factor (PIF) from bovine neurointermediate lobe (NIL) extracts. Acid extracts of bovine NILs stimulated, in a dose-related manner, PRL release from cultured anterior pituitary cells, even after immunoabsorption of endogenous oxytocin from the extract. PIF and PRF activities were semipurified from NIL extracts by Sephadex chromatography and detected by in vitro and in vitro bioassays. The PRF material could be separated from oxytocin by gel sieving and was active in the presence of dopamine in vitro unlike synthetic oxytocin and in cell preparations in which the oxytocin-responsive lactotrophs had been removed by selective cytotoxin cell targeting using an oxytocin-ricin A chain cytotoxic conjugate. The PRF material stimulated PRL secretion in a dose-dependent fashion in conscious male rats after iv injection. The PIF material comigrated on sizing gel chromatography with immunoreactive oxytocin and was active in vitro during dopamine blockade with domperidone and in vivo in the presence of endogenous dopaminergic tone. These data suggest that novel factors present in the NIL might exert physiologically relevant control over lactotroph function and add to the growing literature on the presence of a PRF in the NIL.