Rhesus monkey models of Parkinson's disease were induced by injection of N-methyl-4-phenyl- 1,2,3,6-tetrahydropyridine. Neural firings were recorded using microelectrodes placed in the internal segment of the globus pallidus. The wavelets and power spectra show gradual power reduction during the disease process along with increased firing rates in the Parkinson's disease state. Singular values of coefficients decreased considerably during tremor-related activity as well as in the Parkinson's disease state compared with normal signals, revealing that higher-frequency components weaken when Parkinson's disease occurs. We speculate that the death of neurons could be reflected by irregular frequency spike trains, and that wavelet packet decomposition can effectively detect the degradation of neurons and the loss of information transmission in the neural circuitry.