Summary: During fictive locomotion in the adult decerebrate cat, motoneuron excitability is increased by a hyperpolarization of the threshold potential at which an action potential is elicited (Vth). This lowering of Vth occurs at the onset of fictive locomotion, is evident for the first action potential elicited and is presumably caused by a neuromodulatory process. The present study tests the hypothesis that the monoamines serotonin (5-HT) and noradrenaline (NA) can hyperpolarize neuronal Vth. The neonatal rat isolated spinal cord preparation and whole-cell recording techniques were used to examine the effects of bath applied 5-HT and NA on the Vth of spinal ventral horn neurons. In the majority of lumbar ventral horn neurons, 5-HT (13/26) and NA (10/16) induced a hyperpolarization of Vth ranging from -2 to -8 mV. 5-HT and NA had similar effects on Vth for individual neurons. This hyperpolarization of Vth was not due to a reduction of an accommodative process, and could be seen without changes in membrane potential or membrane resistence. These reveal a previously unknown action of 5-HT and NA to hyperpolarize Vth of spinal neurons, a process that would facilitate both neuronal recruitment and firing.