Therefore, these animals were trained to receive the injections readily. All animals behavior was recorded using a digital video recorder, which helped the monitoring. The routine veterinary care was done by professional keepers and veterinarians. In order to follow the evolution of PD symptoms and assess the chronic effects of L-Dopa treatments on PD, the daily total PD scores were obtained from the recordings during the MPTP induction period and the first one-hour observations during the L-Dopa treatments. All four parts of each of those recordings were scored and then averaged to obtain the daily total score. During the one-month LDopa-treatment period, the difference in the daily total scores between group I and group II was assessed by a independent t-test and analysis of variance analysis. To evaluate the acute effects of both the L-Dopa and morphine treatments on each individual PD symptom, the second postadministration one-hour recording for L-Dopa and morphine treatments were evaluated. The first 15-min section of each recording was excluded to account for immediate drug metabolism and the three remaining 15-min sections were evaluated. The change of each PD symptom was calculated individually using the same comparisons between the scores before and after the drug administration. Morphine can be endogenously synthesized and can increase DA firing rate through disinhibition. In addition, upregulation of neuronal endogenous morphine-like compound immunoreactivity was found in human PD. These facts suggest morphine may have potential therapeutic relevance in PD treatment as dopaminergic neuron loss is related to Parkinson’s disease etiology. However, the use of morphine as a potential therapeutic for PD has not been reported. In this study, an initial investigation into the acute effects of exogenous morphine on PD symptoms was carried out. A rhesus macaque chronic PD model was established through MPTP intoxication. The evolution of PD symptoms were observed to be consistent with other progressive PD models. Following which the animals were treated with either L-Dopa or morphine, four PD symptoms, tremor, bradykinesia, imbalance and defensive reaction, were found to be differentially affected by either morphine or L-Dopa. However, we would concentrate on the changes of tremor and bradykinesia as the two symptoms were the primary PD symptoms. Most notably, morphine treated PD monkeys displayed significant improvements in tremor. This beneficial effect of morphine has not been reported before and indicates a potential therapeutic effect of morphine in PD treatment. Interestingly, the therapeutic effect of morphine was different than that of L-Dopa, the classical PD symptomatic therapy. On the first day of L-Dopa treatment, L-Dopa-treated monkeys displayed temporary improvements on bradykinesia, which was not witnessed in the morphine-treated monkeys. However, LDopa did not display the same improvement that morphine treatment had on tremor. It is noteworthy that the L-Dopa chronically treated monkeys displayed lasting improvements on bradykinesia in comparison to acute morphine or acute L-Dopa treatments.