Many studies have examined motoneurons (MNs) in amyotrophic lateral sclerosis (ALS), but few focus on the premotor interneurons that drive or inhibit MN activity. Our hypothesis was that inhibitory interneurons contribute to dysfunction. To this end, we directly assessed ventrolateral glycinergic neurons (a population that includes Renshaw cells and other classes of inhibitory interneurons) from SOD1G93A x GlyT2eGFP mice at postnatal day (P)6-10 using whole cell patch clamp, immunohistochemistry and anatomical reconstructions. Reconstructed SOD1 interneurons have smaller somata but larger dendrites than WT interneurons. Whole cell patch clamp was performed on SOD1 glycinergic interneurons (n = 25), which show significant dampening of intrinsic excitability compared to controls (n = 34), including a more depolarized threshold for action potentials (APs), more depolarized persistent inward currents (PICs). SOD1 interneurons also had shortened after-spike afterhyperpolarizations (AHPs). Since inhibitory interneurons are a heterogenous population, immunohistochemistry for calbindin was performed to identify Renshaw cells (RCs). In the subset of confirmed RCs, threshold, PIC onset and PIC peak were depolarized and morphological changes in dendrites and somata remained in SOD1RCs (n=6) compared to controls (n=7). A “random forest” statistical model estimated Renshaw cells made up 80% of the interneurons recorded in this study. In RCs predicted by this model, SOD1 RCs showed similar changes to what was observed. In predicted nonRC glycinergic interneurons, the model SOD1 interneurons were more excitable, with larger PICs. Thus, in SOD1 mice glycinergic inhibitory interneurons (both RCs and nonRCs) show very early perturbations in their activity, and likely contribute to dysfunction.