It has been shown that the excitability of cat spinal motoneurons increases during fictive locomotion by lowering the voltage threshold (Vth) for action potential production (Krawitz et al. 2001, J Physiol 532, 271-281). This facilitation of motoneuron recruitment is not directly linked to phasic depolarizations during the step cycle. Rather, it appears to be a state-dependent enhancement of motoneuron excitability, presumably due to a neuromodulatory system. We have also shown that exogenously applied serotonin is able to hyperpolarize the Vth of spinal neurons in the neonatal rat in the absence of locomotor-activity (Fedirchuk 2001, SFN Vol 27, 714.17). The goals of the present series of experiments are to show that activation of locomotor circuitry can cause a lowering of Vth in spinal motoneurons of the neonatal rat, and determine if serotonin is the endogenous neurotransmitter mediating this phenomenon. Experiments were conducted on neonatal (P0-P5) rats that were anaesthetized and decerebrated. The b...