Spastic (spa), spasmodic (spd), and oscillator (ot) mice have naturally occurring glycine receptor (GlyR) mutations, which manifest as motor deficits and an exaggerated “startle response.” Using whole-cell recording in hypoglossal motoneurons, we compared the physiological mechanisms by which each mutation alters GlyR function. Mean glycinergic miniature IPSC (mIPSC) amplitude and frequency were dramatically reduced (>50%) compared with controls for each mutant. mIPSC decay times were unchanged in spa/spa (4.5 ± 0.3 vs 4.7 ± 0.2 ms), reduced in spd/spd (2.7 ± 0.2 vs 4.7 ± 0.2 ms), and increased in ot/ot (12.3 ± 1.2 vs 4.8 ± 0.2 ms). Thus, in spastic, GlyRs are functionally normal but reduced in number, whereas in spasmodic, GlyR kinetics is faster. The oscillator mutation results in complete absence of α1-containing GlyRs; however, some non-α1-containing GlyRs persist at synapses. Fluctuation analysis of membrane current, induced by glycine application to outside-out patches, showed that mean single-channel conductance was increased in spa/spa (64.2 ± 4.9 vs 36.1 ± 1.4 pS), but unchanged in spd/spd (32.4 ± 2.1 vs 35.3 ± 2.1 pS). GlyR-mediated whole-cell currents in spa/spa exhibited increased picrotoxin sensitivity (27 vs 71% block for 100 μm), indicating α1 homomeric GlyR expression. The picrotoxin sensitivity of evoked glycinergic IPSCs and conductance of synaptic GlyRs, as determined by nonstationary variance analysis, were identical for spa/spa and controls. Together, these findings show the three mutations disrupt GlyR-mediated inhibition via different physiological mechanisms, and the spastic mutation results in “compensatory” α1 homomeric GlyRs at extrasynaptic loci.