Motoneurons in the isolated spinal cord of newborn rats (1- to 6-day-old) were visualized using infrared videomicroscopy. Whole-cell recordings were performed from the neurons under observation. Stimulation of a sciatic nerve which adjoined ventral roots elicited postsynaptic currents (PSCs) in 28 out of 88 motoneurons. When membrane potentials were changed, some PSCs reversed at around -70 mV, which was compatible with the chloride equilibrium potential calculated. Thus, they were considered to be recurrent inhibitory, namely Renshaw inhibition. On the other hand, we detected PSCs which reversed at +3.3 mV on average. They were interpreted as excitatory based on the level of the reversal potential which was similar to that of orthodromic excitatory PSCs. To determine the origin of the antidromic excitatory inputs, the effect of d-tubocurarine (10 microM) on the PSCs was examined. In three out of five motoneurons, the excitatory currents were eliminated. Therefore, it was concluded that the excitatory inputs, if not all, are mediated via the axon collaterals. Furthermore, it was found that the locations of motoneurons receiving the recurrent inhibitory and the excitatory PSCs were different in the spinal cord.