The effect of subchronic treatment with l-dopa/carbidopa or l-selegiline on striatal dopamine transporters (DAT) was examined in patients with idiopathic Parkinson's disease with SPECT (single photon emission computed tomography) using [123I]β-CIT (2β-carbomethoxy-3β-[4-iodophenyl]tropane) as the radiotracer. Patients who were not currently being treated with these medications were given either 750 mg l-dopa/carbidopa per day (n = 8) or 10 mg l-selegiline per day (n = 8). [123I]β-CIT imaging was performed three times in each patient: at baseline before treatment, while on medication and after 4–6 weeks of drug treatment, and following withdrawal from medication (approximately 1 week for l-dopa/carbidopa and 9 weeks for l-selegiline). Comparison of scans 2 and 3 provided a measure of drug occupancy of the [123I]β-CIT binding site; comparison of scans 1 and 2 provided a measure of both up- or downregulation of DAT levels and drug occupancy following subchronic drug treatment. DAT levels were assessed from an image acquired approximately 22 hours after radiotracer injection as a ratio of regional brain activities: (striatum − occipital)/occipital. Striatal DAT levels were not significantly different when any two of the three scans were compared for both drug treatments. These results suggest that typical clinical doses of l-dopa/carbidopa and l-selegiline do not induce significant occupancy of the [123I]β-CIT binding site and that 4–6 weeks of treatment causes no significant modulation of DAT levels. These results support the validity of measuring DAT levels with [123I]β-CIT without the need to withdraw patients from medication treatment.