Previous brain imaging studies have demonstrated a seasonal difference of serotonin transporter (SERT) binding in the human brain. However, the results were somewhat contradictory. We conducted test-retest study with single photon emission computed tomography (SPECT) with super(123I-ADAM as ligand in 28 healthy subjects. ten of the subjects were studied within 1 month, whereas 18 were randomly assigned to be studied over a period of up to 1 year. The primary measure was the specific uptake ratio (SUR). Regions of interest included the midbrain, thalamus, putamen and caudate. The intra-class correlation coefficient (ICC) was 0.52-0.94 across different brain regions over 1 month, whereas the ICC was - 0.24 - 0.63 over a 1 -year period. The 1 -month variability ranged from 6.5 +/- 5.1% to 12.5 +/- 10.6% across different brain regions, and the 1 -year variability ranged from 16.5 +/- 9.6% to 41.9 +/- 35.5%. The Kruskal-Wallis test revealed a significant difference of variability across months. The Wilcoxon Signed Ranks Test showed the SUR between test-retest scans was of borderline significance. Curve fitting, using a 4th degree polynomial model, revealed a significant circadian correlation between the variability and interval of test-retest measurements. Our findings demonstrate the test-retest reproducibility of ) super(1)23I-ADAM in different time periods and suggest that circadian variation of SERT levels in the human brain might exist.