Carbon-11-raclopride has been successfully utilized with PET to assess changes in endogenous dopamine concentration after pharmacological intervention in the living baboon brain. For similar studies to be done in humans, measurements of 11C-raclopride with no intervention need to be reproducible. In order to test the reproducibility (test-retest) of 11C-raclopride binding in the human brain, we performed repeated studies on two different days. Studies were done in five normal controls with no pharmacological intervention. Time-activity (%dose/cc) curves for 11C-raclopride in the basal ganglia (BG) and cerebellum (CBL) were highly reproducible with an average difference in peak uptake for repeated studies in the same individual of 4%. The BG to CBL ratio for the average activity concentration between 30 and 60 min showed differences that ranged from -7% to 8% between the repeated studies. Graphical analysis to obtain the distribution volume revealed intrasubject values that ranged from -9% to 7% for the ratio of the distribution volume in BG to that in CBL. These studies demonstrate that in order to use 11C-raclopride to measure an individual's change in relative dopamine concentration secondary to pharmacological or behavioral intervention, a change in striatal 11C-raclopride binding in excess of 10% is required.