Levodopa (L-DOPA), the metabolic precursor of dopamine, is widely used as a pharmacological agent for the symptomatic treatment of Parkinson's disease. However, long-term L-DOPA use results in abnormal involuntary movements such as dyskinesias. There is evidence that abnormal cell signaling in the basal ganglia is involved in L-DOPA-induced dyskinesia. The subthalamic nucleus (STN) plays a key role in the circuitry of the basal ganglia and in the pathophysiology of Parkinson's disease. However, the contribution of the STN to L-DOPA-induced dyskinesias remains unclear. The objective of this work was to study the effects of acute or chronic systemic administration of L-DOPA to adult rats with a unilateral 6-hydroxydopamine (6-OHDA) lesion of dopamine neurons on c-fos expression in the STN and test the hypothesis that these effects correlate with L-DOPA-induced dyskinesias. c-fos mRNA expression was measured in the STN by in situ hybridization histochemistry at the single cell level. Our results confirm earlier evidence that the chronic administration of L-DOPA to rats with a unilateral 6-OHDA lesion increases c-fos expression in the STN. We also report that c-fos expression can be increased following an acute injection of L-DOPA to 6-OHDA-lesioned rats but not following a chronic injection of L-DOPA to sham-operated, unlesioned rats. Finally, we provide evidence that the occurrence and severity of dyskinesia is correlated with c-fos mRNA levels in the ipsilateral STN. These results suggest that altered cell signaling in the STN is involved in some of the behavioral effects induced by systemic L-DOPA administration.