Subthalamic nucleus single-unit recordings were undertaken before and during pedunculopontine [corrected] nucleus-stimulation at clinically relevant frequency (25 Hz) in six patients with Parkinson's disease. Pedunculopontine [corrected] nucleus stimulation changed the firing activity of almost every subthalamic nucleus cell (44/48) by decreasing the ongoing discharge in bursting subthalamic nucleus neurons (-62.1%) and exciting irregular (+63.2%) and tonic/regular discharges (+20.1%). These conflicting data challenge the definition of pedunculopontine [corrected] nucleus as a new target area for Parkinson's disease. If the modulation of subthalamic nucleus bursting units may corroborate the pedunculopontine [corrected] nucleus therapeutic role, the simultaneous excitatory influence during nonbursty patterns might interfere with a favorable outcome on motor signs. As a result, the implantation of pedunculopontine [corrected] nucleus alone may be hazardous while the association of subthalamic nucleus plus pedunculopontine [corrected] nucleus seems to be reasonable.