Impaired glycinergic inhibition causes human hyperekplexia, and may be involved in the pathogenesis of movement disorders associated with uremia, spinal cord lesions, DDT poisoning, and tetanus. Three autosomal recessive mutant mouse strains with single-gene mutations affecting either the alpha 1 (spasmodic and oscillator) or beta (spastic) subunits of the glycine receptor were studied. Serial videotaped examinations assessed the severity of hyperkinetic features. Homozygote oscillator mice appeared normal until postnatal day (P) 11-14, when decreased exploratory movements, spastic gait, stimulus-induced myoclonic bouts, rigidity, and tremor were noticeable. All symptoms gradually worsened until death by P21-P23. In contrast, spastic and spasmodic mice were most severely affected by the 3rd-5th week of life and had a lessening of symptom severity in adulthood. Within each mutant strain, there was marked interanimal variation of severity of the other motor abnormalities, possibly because of stochastic variability in developmental processes. These mutants represent good animal models for elucidation of molecular and cellular issues regarding the glycine receptor and for the study of pathogenetic mechanisms of movement disorders.