GABA and glycine are excitatory in the immature spinal cord and become inhibitory during development. The shift from depolarizing to hyperpolarizing inhibitory post-synaptic potentials (IPSPs) occurs during the perinatal period in the rat, a time window during which the projections from the brainstem reach the lumbar enlargement. In the present study we investigated the effects of suppressing influences of the brain on lumbar motoneurons during this critical period for the negative shift of the reversal potential of IPSPs (E_IPSP). The spinal cord was transected at the thoracic level on the day of birth [postnatal day 0 (P0)]. E_IPSP, at P4-7, was significantly more depolarized in cord-transected than in cord-intact animals (E_IPSP above and below resting potential, respectively). E_IPSP at P4-7 in cord-transected animals was close to E_IPSP at P0-2. K-Cl cotransporter KCC2 immunohistochemistry revealed a developmental increase of staining in the area of lumbar motoneurons between P0 and P7 in cord-intact animals; this increase was not observed after spinal cord transection. The motoneurons recorded from cord-transected animals were less sensitive to the experimental manipulations aimed at testing the functionality of the KCC2 system, which is sensitive to [K⁺]_o and blocked by bumetanide. Although bumetanide significantly depolarized E_IPSP, the shift was less pronounced than in cord-intact animals. In addition, a reduction of [K⁺]_o affected E_IPSP significantly only in cord-intact animals. Therefore, influences from the brainstem may play an essential role in the maturation of inhibitory synaptic transmission, possibly by upregulating KCC2 and its functionality.