Although substantia nigra reticulata (SNR) neurons fire bursts of action potentials during normal movement, excessive burst firing correlates with symptoms of Parkinson's disease. A major excitatory output from the subthalamic nucleus (STN) to the SNR is thought to provide the synaptic impetus for burst firing in SNR neurons. Using patch pipettes to record from SNR neurons in rat brain slices, we found that a single electrical stimulus delivered to the STN evokes a burst of action potentials. Under voltage-clamp conditions, STN stimulation evokes a complex EPSC that is comprised of an initial monosynaptic EPSC followed by a series of late EPSCs superimposed on a long-lasting inward current. Using varied stimulation frequencies, we found that the initial EPSC was significantly reduced or abolished after 2 s of 50-100 Hz STN stimulation. However, only 4 s of 1 Hz stimulation was required to abolish the late component of the complex EPSC. We suggest that differential effects of repetitive STN stimulation on early and late components of complex EPSCs may help explain the frequency-dependent effects of deep brain stimulation of the STN that is used in the treatment of Parkinson's disease.