Istradefylline reverses CGS21680-induced and reserpine-induced catalepsy with ED50 of 0.05 mg/kg and 0.26 mg/kg, respectively. Istradefylline is over 10 times as potent in these models compared to other adenosine antagonists and dopamine agonist drugs. Administration of Istradefylline in combination with L-dopa (50 mg/kg) exerts prominent effects on haloperidol-induced and reserpine-induced catalepsy. Oral administration of Istradefylline at 10 mg/kg to MPTP-treated common marmosets produces an increase in locomotor activity to approximately twice that of control and improves motor disability. Administration of Istradefylline (10 mg/kg, po, 90 minutes before SKF80723/quinpirole/L-DOPA) in combination with SKF80723 (1 mg/kg, ip), quinpirole (0.06 mg/kg ip), or L-DOPA (2.5 mg/kg po) produces a significant additive effect on locomotor activity and improvement of motor disability but not dyskinesia. In the MPTP mice model, Istradefylline significantly attenuates striatal dopamine depletion under various conditions. Pretreatment with Istradefylline (3.3 mg/kg, i.p.) before a single dose of MPTP attenuates the partial dopamine and DOPAC depletions measured in striata 1 week later. Oral administration of Istradefylline protects against the loss of nigral dopaminergic neuronal cells induced by 6-hydroxydopamine in rats, and prevents the functional loss of dopaminergic nerve terminals in the striatum and the ensuing gliosis caused by MPTP in mice.  Chronic Istradefylline treatment does not improve the reversal deficits in dopamine-depleted rats. The tremulous jaw movements induced by pimozide are significantly reduced by co-administration of either Istradefylline or tropicamide. Pimozide-induced increases in ventrolateral striatal c-Fos expression are reduced by a behaviorally effective dose of Istradefylline, in contrast to tropicamide by which c-Fos expression in pimozide-treated rats is actually increased.