The nature of the synaptic drive from the locomotor spinal network onto the motoneurons was studied in the newborn rat. For this purpose, an in vitro isolated spinal cord preparation of newborn rat was used. The recording chamber was partitioned with Vaseline walls to separate the L1/L2 lumbar segments, in which the spinal locomotor network is located, from the motoneurons in the lower lumbar segments. Locomoter- like activity was induced by bath-applying a mixture of serotonin and NMDA to segments L1/L2. In this way, the synaptic activity could be modified at the lower lumbar level without affecting the motor pattern. The drive elicited onto the motoneurons during sequences of locomoter- like activity, which was monitored by performing intracellular recordings, consisting of an inhibitory component followed by an excitatory component. The inhibitory synaptic volley was reversed at a membrane potential of --60 mV with K acetate electrodes, whereas it was shifted toward positive values with KCl electrodes. The glycinergic blocker strychnine, bath-applied to segments L3/L5, blocked the inhibitory drive without affecting the rhythmic activity, whereas it disrupted the locomoter-like activity when bath-applied to segments L1/L2. The inhibitory part of the drive was more sensitive than the excitatory part to changes in the membrane potential. The excitatory phase was mixed and consisted of an NMDA and a non-NMDA component, which were sensitive to 2-amino-5-phosphonovaleric acid and 6-cyano-7- nitroquinoxaline-2,3-dione, respectively. It was concluded that the locomotor network located in segments L1/L2 sends a biphasic projection to the various groups of motoneurons located along the lumbar spinal cord.