The mechanism by which deep brain stimulation (DBS) of the subthalamic nucleus (STN) achieves its effects in Parkinson's disease (PD) is not known. In animal models of PD, stimulation and lesioning of the STN have some effects which are the same, but others which differ, in reversing cellular and behavioral changes induced by dopamine depletion. We compared the effects of short-term STN stimulation and lesions upon extracellular levels of dopamine and metabolites using in vivo microdialysis of the dorsal striatum of awake, intact and unilateral 6-hydroxydopamine (6OHDA)-lesioned rats. STN stimulation in control rats decreased striatal dopamine levels and caused a relative increase in dopamine metabolism, as expressed by HVA/dopamine and DOPAC/dopamine ratios. This suggests an increase in both vesicular dopamine release (metabolized to HVA), and release from the cytoplasmic dopamine pool (metabolized to DOPAC). STN lesions in control rats increased the HVA/dopamine ratio, also suggesting a relative increase in vesicular dopamine release. These results indicate that STN stimulation and lesioning can affect striatal dopamine metabolism in the intact system. In 6OHDA-lesioned rats at baseline, metabolic ratios were markedly decreased as compared with controls. STN lesions of 6OHDA-lesioned rats did not affect relative metabolic ratios as compared with baseline levels. In 6-OHDA-lesioned rats, STN stimulation decreased extracellular levels of dopamine, and, to a greater extent, metabolites, resulting in a decrease in metabolic ratios. This further decrease in dopamine turnover with STN stimulation would serve to maintain dopamine levels in the dopamine-depleted striatum, and may account for the therapeutic benefit of DBS in Parkinson's disease.