We studied the serotonin effect on monosynaptic reflex potentials (MSR) of spinal motorneurons in the decerebrated rats in control and after intraperitoneal administration of serotonin precursor – 5-Hydroxytryptophan (5-HTP). MSR of motorneurons in the lumbar spinal cord were registered using electrical stimulation of dorsal root of the 5th lumbar section. During stimulation physiological saline or 5-hydroxytryptophan was injected intraperitoneally. In comparison with average amplitude of the control MSR there were registered significant increase in amplitudes of the MSR (169% and +172%, P <0,001) in animals with injection 5-HTP. These data suggest that serotonin release after 5-HTP administration leads to activation of motorneurons in the lumbar spinal cord. The mechanism of this activation may be related to the weakening of the inhibitory control of interneurons in the transmission pathways of the excitatory influences from muscle afferent to motorneurons and to the postural (antigravity) reflex reactions which necessary for the initiation of locomotion.