Intracellular recordings were made from 25 antidromically identified motoneurons (MNs) in the transverse thoracolumbar spinal cord slices (500 microns) of neonate rats (7-16 d). In more than 80% MNs, depolarization potentials (EPSP) with latency of 1.2 +/- 0.2 ms could be evoked by focal stimulation (0.1 ms, 1-20 V, 0.1 Hz) of ventrolateral funiculus. When recorded at resting potential of -70 +/- 6 mV, the time-to-peak, amplitude, half-decay time and duration of EPSPs to suprathreshold stimuli were respectively, 2.6 +/- 0.4 ms, 13 +/- 3 mV, 5.3 +/- 1.6 ms and 31 +/- 8 ms. The EPSPs were graded in consummate to stimulus intensity and upon reaching threshold depolarization, action potentials were initiated. Relatively constant latency of EPSPs was observed in spite of varying stimulus frequency from 0.1 to 5 Hz. Upon increasing stimulation frequency > 20 Hz, EPSPs began to decrease and finally disappeared. In 3 MNs tested, the mean reversal potential of EPSPs was -8 mV. Low Ca/high Mg solution consistently attenuated but Mg-free solution enhanced the EPSPs. EPSPs were reversibly depressed by kynurenic acid (0.5-1 mmol/L) and partially inhibited by ketamine (50-100 mumol/L). The findings of the present investigation suggest that MNs are excited by the descending fibers in ventrolateral funiculus possibly via excitatory amino acid transmitters.