An exaggerated startle response caused by mutations of the alpha 1 subunit gene of the inhibitory glycine receptor (GLRA1) is the key symptom of human hyperekplexia or startle disease. The recessive mouse mutant spasmodic (spd) carries a missense mutation in the corresponding murine Glra1 gene which reduces the affinity of agonists for the mutant receptor. This mutant has been regarded as an animal model with which to investigate the molecular basis of hyperekplexia and related motor disorders. The recessive mouse mutant spastic (spa) carries an insertional mutation in the glycine receptor beta subunit gene (Glrb) that results in aberrant splicing and, consequently, in a reduced number of functionally intact receptors. The resulting phenotype is similar to that of spasmodic. This study measured the acoustic startle response of spasmodic and spastic mice under different stimulus conditions, in order to test for sensorimotor processing deficits in these animals. Both mutants show increased startle responses to acoustic stimuli of different intensities compared with wild-type animals.