The present study was designed to: 1) morphologically characterize cat glutamate and GABAergic synaptic terminals in lamina IX in the intact spinal cord at the electron microscopic level using postembedding immunochemical techniques and .2), begin an analysis of how the synaptic architecture of glutamate and GABAergic terminals changes after an ipsilateral spinal cord hemisection. The present study shows that glutamate immunoreactive terminals are characterized by a wide synaptic cleft, asymmetric synaptic membrane densities and spherical synaptic vesicles. Most of the glutamatergic terminals are presynaptic to small or medium size dendrites. In contrast, GABAergic terminals display typical pleomorphic synaptic vesicles, a narrow synaptic cleft and a symmetrical membrane density. Qualitative analysis indicated that 13-17 months after hemisection, the length of the synaptic active zones in both glutamatergic and GABAergic terminals ipsilateral to hemisection is longer than those observed in the terminals contralateral to hemisection orfin normal control cats. Furthermore, the perimeters of both dendrites and either glutamate or GABA immunoreactive terminals are longer on the hemisected side compared with those observed in the nonhemisected side of the spinal cord. The results are important for complete understanding of the mechanisms which underlie locomotor recovery in mammals following spinal cord injury.