Spasticity may result in part from segmental spinal disinhibition. We determined the content and specific activity of glycine (the putative neurotransmitter thought to mediate spinal postsynaptic inhibition) and serine (the probable precursor of glycine) in feline spastic spinal cord following the intra-aortic administration of two labelled precursors of glycine—14C-D-glucose and 14C-L-serine. The specific activities of both glycine and serine were significantly reduced in the ventromedial, central, and dorsal spinal gray matter in spastic animals. Glycine content remained at control values but serine content increased in spastic spinal cord. This study suggests that glycine turnover decreases in spasticity, owing to its diminished release, and supports neurophysiologic evidence of a decrement in postsynaptic inhibition.