Inhibitory synapses with large and gephyrin-rich postsynaptic receptor areas are likely indicative of higher synaptic strength. We investigated the presynaptic inhibitory neurotransmitter content (GABA, glycine, or both) and the presence and subunit composition of GABAA and glycine postsynaptic receptors in one example of gephyrin-rich synapses to determine neurochemical characteristics that could also contribute to enhance synaptic strength. Hence, we analyzed subunit receptor expression in gephyrin patches located on Renshaw cells, a type of spinal interneuron that receives powerful excitatory and inhibitory inputs and displays many large gephyrin patches on its surface. GABAA and glycine receptors were almost always colocalized inside Renshaw cell gephyrin clusters. According to the subunit-immunoreactivities detected, the composition of GABAA receptors was inferred to be either α3β(2or3)γ2, α5β(2or3)γ2, α3α5β(2or3)γ2 or a combination of these. The types of neurotransmitters contained inside boutons presynaptic to Renshaw cell gephyrin patches were also investigated. The majority (60–75%) of terminals presynaptic to Renshaw cell gephyrin patches contained immunocytochemical markers for GABA as well as glycine, but a proportion contained markers only for glycine. Significantly, 40% of GABAA receptor clusters were opposed to presynaptic boutons that contained only glycinergic markers. We postulate that GABA and glycine corelease, and the presence of α3-containing GABAA receptors can enhance the postsynaptic current and contribute to strengthen inhibitory input on Renshaw cells. In addition, a certain degree of imprecision in the localization of postsynaptic GABAA receptors in regard to GABA release sites onto adult Renshaw cells was also found. J. Comp. Neurol. 444:275–289, 2002. © 2002 Wiley-Liss, Inc.