Statistical analyses (autocorrelation and first-order interstimulus interval) were conducted on the spontaneous activity of over 420 subthalamic neurons recorded in 5 groups (control, large globus pallidus kainic acid lesion, partial globus pallidus kainic acid lesion, partial globus pallidus ibotenic acid lesion and neostriatal lesion) of anesthetized rats. Cross-correlation and peristimulus time histogram (to frontal motor cortex stimulation at 0.7 mA) analyses were conducted on pairs (n = 58) of subthalamic neurons recorded simultaneously on a single microelectrode. Lesion of the globus pallidus increased spontaneous firing rate as compared to controls and shifted the pattern of spontaneous activity from either a regular or irregular pattern to a markedly bursting pattern. Neostriatal lesion reduced firing rate and reduced the likelihood of highly regular firing. In control, neostriatal and partial lesioned animals, approximately 1 in 3 pairs of neurons showed correlated firing. The correlations were joint increased probabilities of firing over intervals of 200-400 ms, suggesting a shared excitatory input. No short-interval (less than 10 ms) correlations were seen. Large globus pallidus lesion increased the likelihood of correlated firing (12 of 16 pairs). In all groups of animals the peristimulus time histograms (PSTHs) to motor cortex stimulation were more similar than would be expected by chance and pairs of neurons showed the same increases in response following globus pallidus lesion. Thus adjacent neurons share common cortical inputs and responsiveness to those inputs. These changes indicate that the globus pallidus influences the spontaneous firing rate and pattern of subthalamic neurons as well as the degree of correlated firing of adjacent neurons.