Prolactin (PRL) is an anterior pituitary hormone important for fertility and reproductive success. PRL is primarily regulated by inhibition from hypothalamic dopamine (DA) and DA levels must typically drop in order for large amounts of PRL to be released. Endogenous opioids are believed to be necessary for the inhibition of DA that allows the surge in PRL secretion during pregnancy, delivery, and lactation. During early rodent pregnancy or pseudopregnancy induced by cervical stimulation (CS), female rats exhibit twice-daily surges in PRL release, one nocturnal and one diurnal surge. We show that i.c.v norbinaltorphimine (nor-BNI), a kappa opioid receptor antagonist, blunts the nocturnal PRL surge in cervically-stimulated ovariectomized rats (OVX-CS), similar to a previous report in intact, pregnant animals.  This result suggests that endogenous dynorphin, perhaps coming from the KNDy neurons of the arcuate nucleus, is a factor in the control of PRL release during pregnancy and pseudopregnancy. On the afternoon of proestrus in the rat, PRL levels, along with many other hormones, rise to help prepare the female for mating and potential pregnancy. This single diurnal rise in PRL can be reproduced in OVX rats treated with replacement estradiol (OVE). In OVE rats, nor-BNI administration into the lateral ventricle immediately before the expected PRL surge did not change DA and PRL profiles, suggesting that the diurnal estradiol-induced surge is generated by a different mechanism than the nocturnal CS-induced PRL surge. Future tests will explore the effect of kappa opioid blockade on the diurnal PRL surge in OVX-CS animals. These results will provide crucial information on the mechanisms employed in the generation PRL surges induced by cervical stimulation and estradiol.