ABSTRACT - Parturition in mammals occurs in the context of sensory, neurochemical, and endocrinological factors that are orchestrated and timed so that maternal behavior and the object of the behavior, the neonate, "emerge" almost simultaneously. Among the factors found to be important for the suppression of pain during delivery as well as for the emergence of caretaking behavior toward the young, are changes in endogenous opioid activity in the central nervous system. In most mammalian species, these changes are likely initiated by sensory events arising in the distended reproductive tract and abdominal musculature, and are modified by the parturitional endocrine milieu and substances ingested in amniotic fluid and placenta (e.g., Placental Opioid-Enhancing Factor, or POEF). In addition, ingestion of afterbirth material may decrease the probability that the vaginal/cervical sensory stimulation arising during delivery will trigger pseudopregnancy, a condition that decreases, if not eliminates, the likelihood of fertilization in the postpartum estrus. The research described herein primarily focuses on elucidating the manner in which POEF modulates opioid antinociception, and otherwise participates in opioid-mediated parturitional events.