Spontaneous sleep movement disorders in 5 cats and 3 dogs were studied. Objective abnormalities during sleep were confirmed by electrographic or behavioral monitoring techniques in all animals. The cause of the disorder was CNS disease in 3 animals. A cause was not discovered in the other 5 animals, although at necropsy, 2 were found to have thyroid tumors. Many pharmacologic treatments were ineffectual, but trials with the benzodiazepine tranquilizer, clonazepam, suggested it will prove useful in controlling violent movements during rapid-eye-movement sleep.