New mothers undergo dramatic endocrine changes that elicit widespread neural plasticity.  The birth and development of new brain cells has been studied in several regions of the peripartum forebrain where reproductive state and maternal interactions with pups affect cytogenesis, but sites in the midbrain have never been reported.  In these studies, we investigated cytogenesis in the dorsal raphe, the brain’s main source of serotonin.  Serotonin affects lactation and various aspects of postpartum socioemotional behaviors, so neuroplasticity within the dorsal raphe could play a vital role for new mothers.  Using bromodeoxyuridine (BrdU; 150 mg/kg) to label mitotic cells in virgin, pregnant, and postpartum rats, we recently discovered that cells born one week after parturition were less likely to survive than cells born during pregnancy.  Next, we determined whether pup presence might contribute to the postpartum reduction in cytogenesis.  Using BrdU to label mitotic cells in postpartum females with or without their pups, we found that interacting with pups significantly reduced survival of new cells in the dorsal raphe.  To determine if pup presence reduced cell survival by increasing the dams’ circulating corticosterone levels, an ongoing experiment is assessing whether adrenalectomy increases cytogenesis in the dorsal raphe of dams with their pups and whether replacement of corticosterone prevents this effect.  Lastly, to determine possible functional significance of this change in cytogenesis, we are currently examining serotonin content in the dorsal raphe across reproductive state and in dams with or without pups.  Research into neuroplasticity in the serotonin systems underlying the normal postpartum changes in behavior and physiology could help identify novel targets for ameliorating postpartum emotional disorders and facilitate interactions between mothers and their infants.