Behavioral and electromyographic observations in monkeys with intercollicular brain stem transection revealed varying but nonfixed posture and tone, with flaccidity and subtle, intermittent claspknife spacticity in both flexors and extensors, despite brisk tendon jerks. Extremities remained wherever passively placed. Reflexes characterizing such preparations included lengthening and shortening reactions, symmetrical and asymmetrical tonic neck and crossed extensor reflexes, flexor withdrawal responses, and reflex standing. Extensor posturing, usually associated with decerebrate preparations, was manifest only as a response to nociceptive stimulation of face or trunk, hypoxia, or passive neck extension; these maneuvers consistently induced an identical sequence of motor responses in neck and limb musculature, with lack of absolute reciprocal inhibition. Therefore, decerebrate extensor responses are not stable passive release phenomena, but are reflexly maintained and driven; hence they are called the "reactive extensor postural synergy" (REPS).