In the present study, the effectiveness of 3 drug regimen on cognitive performance of PD patients was compared. 12 patients who had been using pramipexole, levodopa and amantadine for at least 1 month entered the study and compared with those 12 who had been using trihexiphenidyle, levodopa and amantadine. There was also a control group including 11 patients who had been using only levodopa and amantadine. All 3 groups were asked to answer Montreal Cognitive Assessment in pretest phase. Then patients in experimental groups were asked not to use pramipexole or trihexiphenidyle for 72 hours and then all 3 groups were asked to answer the same questionnaire in post test phase. The results showed that patients who have used pramipexole had better performance in executive functions in post test. The findings suggest that pramipexole in combination with levodopa and amantadine may worsen the executive function in Parkinson's disease; however, there is almost neither adverse nor beneficial effect of trihexiphenidyl in such a combination on cognition in PD patients.