There are two types of posthypoxic myoclonus, acute and chronic. The acute type has not been intensively studied but is likely to be brain stem in origin. Chronic posthypoxic myoclonus is most commonly cortical reflex myoclonus, but reticular reflex myoclonus and exaggerated startle may also occur. These three conditions have characteristic physiological features. In assessing the relevance of an animal model, it would be appropriate to identify the myoclonus with one of these three patterns.