A 7-year-old female baboon presented with an initial episode of mild lameness of the left hindlimb. This clinical complaint was noted 7 weeks after surgery to place a head platform for neuropharmacological and behavioral testing. Over the following 6 days, the baboon became agitated, and her lameness slowly worsened. The baboon appeared bright, alert and responsive, slightly agitated, partially nonweight bearing on the left hindlimb, occasionally dragging the dorsal surface the left foot. A complete physical examination was performed under ketamine (5 to 10 mg/kg, Ketaved; Vedco, Inc., St. Joseph, Mo.) sedation. Her hydration was normal, temperature was 39.9 degrees C, heart rate was 120 beats/min., respiratory rate was 24 breaths/min., and weight was 10.4 kg (pre-surgical weight was 11.5 kg). PL mild swelling in the left knee and crepitus of the left ankle were also noted. Normal femoral pulses were present bilaterally, with both limbs warm to the touch. Toe-pinch withdrawal and anal reflexes were bilaterally normal. Routine husbandry consisted of individual housing in two side-by-side group 4 stainless steel cages in a multisex primate room. Old World Primate Chow (Purina 5049; Purina Mills, Inc., St. Louis, Mo.) was fed twice daily, and water was provided ad libitum via an automatic watering system. Ambient temperature was maintained at 22 +/- 1 degrees C, with relative humidity between 40 and 50%. Environmental enrichment included watching television and playing with rubber dog toys, swings, mirrors, and pick boards. Historical review of prior neuropharmacologic studies showed that the baboon had arrived from another institution and had been used in a study involving gamma amino butyric acid (GABA) and glutamate receptors in the limbic cortex. She had received a head injection platform in July 1994, which was removed 2 years later due to, complications. A new platform was placed at Georgetown University Medical Center. Since placement of the platform, the animal had received three intracranial injections of muscimol (Sigma Chemical, St. Louis, Ma.), a powerful central nervous system (CNS) GABA agonist, at 2-week intervals. These injections were placed into the left, right, and bilateral substantia nigra respectively The bilateral injection was performed 28 days prior to the presenting complaint. Muscimol was given to evaluate change in GABA transmission as a mechanism for protection against epileptic seizures induced in the substantia nigra. After the last injection, the baboon exhibited abnormal motor responses characterized as 30 to 60 min of turning to the right, head tilt to the right, and left-arm choreiform motions. These motions represented a variety of rapid, highly complex jerky movements that appeared well coordinated but were performed involuntarily. Because of these unilateral results in response to the bilateral injection, the research group ran a magnetic resonance image (MRI) brain scan to confirm proper coordinate placement of the platform. Proper injection coordinates were confirmed. This MRI was taken 10 days prior to the initial complaint of hindlimb weakness.