We studied the effect of MPTP on sleep-wakefulness cycle in cats. Five mg/kg n-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) was administered i.p. for 5 consecutive days. Electrocorticographic, electrooculographic and electromyographic recordings were performed before (5 days), during (5 days) and after (14 days) the treatment. Total selective paradoxical sleep deprivation was observed from the first injection. This effect lasted 6-9 days after the last dose, while the relative amount of slow wave sleep increased. The Berg-Fourier analysis showed no significant change in the EEG power spectra of slow-wave sleep during the paradoxical sleep deprivation compared to control period. Recurrence of paradoxical sleep was parallel to the disappearance of the motor symptoms. Histopathological investigation showed neuronal loss mainly in the substantia nigra. Our present study suggests a complex behavioral effect of MPTP.