Iodine-123 labelled 2beta-carbomethoxy-3beta-(4-iodophenyl) (nor-beta-CIT) is an analogue of beta-CIT, which has high affinity to the serotonin transporter. Initial single-photon emission tomography (SPET) studies with [123I]nor-beta-CIT were performed in five healthy volunteers. In addition, its metabolism in plasma was investigated with gradient high performance liquid chromatography. [123I]nor-beta-CIT was prepared by a method which gave a specific radioactivity of more than 180 GBq/micromol. Unchanged [123I]nor-beta-CIT in plasma accounted for 43% and 19% of total radioactivity after 30 and 180 min, respectively. The dynamic SPET studies demonstrated a high and rapid uptake of radioactivity in the brain (6%/ID at 30 min). Highest accumulation was observed in the striatum, the mid-brain and the thalamus. The specific binding in the mid-brain was 33% higher compared with that of [123I]beta-CIT. The high radioactivity in the mid-brain is assumed to represent the accumulation of [123I]nor-beta-CIT in the serotonin transporter-rich regions, which indicates that [123I]nor-beta-CIT might be a potential tracer for visualization of serotonin transporter sites in the human brain with SPET.