Attacks of sustained dystonic postures of limbs and trunk can be initiated by mild environmental stimuli in an inbred line of Syrian hamsters. The trait is determined by an autosomal simple recessive genetic mutation, originally designated by the gene symbol sz, because the abnormal movements were thought to represent epileptic seizures. The attacks, which can be reproducibly initiated by placing the sz mutant hamsters in a new environment, begin with rapid twitches of the vibrissae, flattened ears, and flattened posture of the trunk while walking, followed by facial contortions, rearing, and sustained posturing of trunk and limbs, often resulting in falling over to the side or backwards. In the final stage, the hamsters became immobile, which can last for hours. An increased tone of limbs and trunk muscles can be palpated during the attack. Electromyographical recordings in awake, unrestrained mutant hamsters showed that the onset of the attack coincided with continuous tonic muscle activity and phasic bursts, which were present even when the animals did not move. During the attack, the animals continue to react to external stimuli. Bilateral electroencephalographic (EEG) recordings before and during motor disturbances in sz mutant hamsters showed no abnormalities. The severity of the dystonic syndrome in hamsters is age dependent with a peak at about 30–40 days of age. A score system for grading type and severity of dystonic attack was developed for use in drug activity studies. The severity of the attack was reduced or attacks were completely prevented by diazepam (1–2.5 mg/kg i.p.) and valproic acid (100–400 mg/kg i.p.) in a dose-dependent fashion. The latency to dystonic movements was significantly increased by diazepam but markedly reduced by subconvulsive doses of pentylenetetrazol (40 mg/kg s.c.). Diazepam antagonized the latency-reducing action of pentylenetetrazol in the hamsters. The pathophysiology and pharmacological sensitivity of the dystonic attacks in these animals remain to be further clarified, but the data indicate that the sz mutant hamsters might represent an interesting genetic model for paroxysmal dystonia. In view of these data, we propose that the hamster mutation should be re-named dystonic and that the new gene symbol should be designated dtsz.