OBJECTIVE [(11)C]AZ10419369 is a recently developed 5-HT1B receptor radioligand that is sensitive to changes in endogenous serotonin concentrations in the primate brain. Thus, [(11)C] AZ10419369 may serve as a useful tool in clinical studies of the pathophysiology and pharmacological treatment of diseases related to the serotonin system, such as depression and anxiety disorders. The aim of this study was to evaluate the test-retest reliability of [(11)C]AZ10419369. METHODS Eight men were examined with PET and [(11)C] AZ10419369 twice on the same day. The binding potentials (BPND) of [(11)C]AZ10419369 in selected serotonergic projection areas and in the raphe nuclei (RN) were determined using the simplified reference tissue model, and for comparison also using a wavelet-aided parametric imaging approach. The BPND values obtained from the first and second PET scans were compared by means of descriptive statistics, difference, absolute variability and intraclass correlation coefficient. RESULTS Similar BPND values were obtained with the two methods. The absolute mean differences in BPND between PET 1 and PET 2 were less than 3% in all serotonergic projection regions. Absolute variabilities were low in cortical regions (5 - 7%), low to moderate (7 - 14%) in subcortical regions, but higher (20%) in the RN. CONCLUSIONS The BPND of [(11)C]AZ10419369 is highly reproducible in cortical regions and satisfactory in subcortical projection areas. The variability in the RN is higher. Thus larger sample sizes or larger divergences are required to assess a potential difference between subjects or between experimental conditions in this region.