Individuals in the early stage of Parkinson's disease exhibit cognitive impairments as a result of hemisphere damage. The mesocortical dopamine system, particularly the medial prefrontal cortex (mPFC), is implicated in cognitive functions and is characterized by an asymmetric organization. Oxytocinase activity (OX) is also asymmetrically distributed in the mPFC of normal rats and is involved in cognitive functions. OX was measured in the left and right mPFC of rats with left or right hemi-parkinsonism, induced by intrastriatal injections of 6-hydroxydopamine, and compared with sham controls. These results demonstrated that the striking basal left predominance of OX observed in both the left and the right sham controls was radically disrupted in lesioned animals. The bilateral distribution in lesioned animals was altered differently depending on the injured hemisphere. These results may reflect changes in the enzyme substrates and consequently in the functions in which they are involved. These results may account, in part, for the cognitive abnormalities observed in hemi-parkinsonism.