This study examined medication use among Medicare beneficiaries and dually eligible beneficiaries before and after the implementation of Medicare Part D on January 1, 2006. Nationally representative 2004-2006 data from the Medical Expenditure Panel Survey were used. Two large classes of psychotropic medications (antidepressant and antipsychotic medications) and two large classes of nonpsychotropic medications (lipid-lowering and antihypertensive agents) were examined to determine whether changes in prescription patterns occurred as a result of the implementation of Part D. There was no strong evidence that Part D was associated with large changes in access to medications in the four classes of medications examined here.