To ascertain the extent to which neuronal firing pattern in the subthalamic nucleus (STN) is determined by afferent inputs, a comparison was made between STN neurons recordedin vivo andin vitro (a largely denervated preparation).In vivo, the majority of cells exhibited an irregular firing pattern, although some showed evidence of burst firing. In contrast, all cells had a regular firing patternin vitro. Electrical stimulation of the striatopallidal complexin vivo induced a short latency inhibition in STN neurons, followed by a burst of spikes. These effects could be reproducedin vitro; hyperpolarising pulses gave rist to a slow depolarising potential upon termination, which was accompanied by a burst of action potentials. Hence, the evidence suggests that afferents play an important role in determining the firing pattern of STN neurons. Howerver, the cells also possess intrinsic membrane properties which allow inputs to trigger either single spikes or bursts.