Although it is well accepted that the spinal cord contains a system of interneurons (INs) capable of producing the locomotor rhythm, little is known about their electrophysiological properties. Here we investigate the properties of these neurons and the modulation of their properties by 5-HT and ACh, transmitter substances known to induce locomotor activity. Fluorescent INs were studied using whole cell patch clamp techniques in spinal cord slices from P4-P8 Cfos-EGFP mice (Brownstone et al, SFN Abs 65.8, 2002) following locomotor activity (walking or swimming 60-90 min). The target areas of study included laminae VII, VIII&X from segment T11-L6. The main properties measured from these neurons include input resistance(Ri),time constant, whole cell capacitance, current(Ith) and voltage threshold(Vth), and the frequency-current(f/I) relation. The EGFP labeled cells could be classified into three types based on firing induced by current injection(n=34): single spike (type 1, n=6), brief firing (type 2, n=7), ...