[ 123I]IBZM is a new radioactive labelled ligand which has a high affinity and specificity to D2-dopamine receptors. The in vivo kinetics of [ 123I]IBZM were studied in patients with unilateral and bilateral accentuated idiopathic Parkinson's disease. The uptake in the basal ganglias and the imaging properties of this D2 receptor antagonist as a radiopharmaceutical for SPECT examinations had to be investigated. 5 patients, aged 42–66 years, ( 2 m 3 f ) were examined. Each patient received 185 MBq [ 123I]IBZM intravenously. Blood samples were taken 0–120 min post injection (p.i.) and time activity curves were plotted. Three SPECT examinations were performed (I: 30–50 min; II: 50–70 min; and 70–90 min p.i.). The count rates (counts/pixel) in the basal ganglias and the cerebellum were measured for each SPECT series on transverse slices using the region-of-interest technique. The time-activity curve of [ 123I]IBZM shows a rapid decline in plasma during the first 10 min followed by a plateau until 120 min after injection. The SPECT examinations demonstrate the highest count rate in the basal ganglia during SPECT series III (i.e., 70–90 min p.i.). The side-to-side difference of the count rates were in the range of 3% in four patients, and 10% in one patient. The biokinetic data of [ 123I]IBZM make this substance capable as a radiopharmaceutical for SPECT examinations. The basal ganglia are best visualized 70–90 min p.i., thus [ 123I]IBZM seems to be a promising imaging agent for diseases of the D2-dopaminergic receptor system.