Cypermethrin is a potent pesticide derived from natural pyrethrin of the chrysanthemum plant. Cypermethrin has been known to modulate the blood-brain barrier and induce oxidative stress in rats. The oxidative stresses leading to increased reactive oxygen species generation have been identified within the degeneration of the dopaminergic (DA) neuron. However, in testing cypermethrin for its relationship to the degeneration of DA neurons, an experimental study has not yet been done. This study was designed to investigate the effects of cypermethrin on the DA neurons in the substantia nigra of normal and progressive hemiparkinsonian rats. The degree of degeneration of DA neurons was evaluated by tyrosine hydroxylase (TH) immunohistochemistry and forepaw adjusting step (FAS) test. The administration of cypermethrin (15 and 75 mg/kg/day) to the normal rats for 15 days did not decrease the number of TH-immunopositive (TH-IP) DA neurons in the substantia nigra. However, the low dose (15 mg/kg/day) of cypermethrin enhanced the rate of decline of DA neurons in the substantia nigra of hemiparkinsonian rats at 10 days and 3 weeks (p < 0.05). Also, the number of FAS in cypermethrin-treated hemiparkinsonian rats was reduced more rapidly than that of cypermethrin not-treated hemiparkinsonian rats at 10 days, 3 weeks, and 6 weeks (p < 0.05). These results suggest that cypermethrin per se cannot directly induce the degeneration of DA neurons but can accelerate a toxic effect on the degeneration of DA neurons in the progressive hemiparkinsonian rats.