Abstract 1′-[4-[1-(4-Fluorophenyl)-1 H-indol-3-yl]-1-butyl]spiro[isobenzofuran-1(3H),4′-piperidine] (Lu 28-179) or Siramesine is a σ-ligand with high specificity for σ 2-sites, has been tritiated to directly label these sites in brain. [ 3H]Lu 28-179 bound in a saturable manner to homogenates prepared from rat brain or human cortices with high affinity ( K d=2.2 and 1.1 nM, respectively) and moderate capacity ( B max=50±12 and 26±3 fmol mg tissue −1, respectively). The rank order of affinity for [ 3H]Lu 28-179 binding to homogenates from human and rat brain was: Lu 28-179>ifenprodil=haloperidol>1,3-di(2-tolyl)-guanidine (DTG)>(−)pentazocine≫(+)pentazocine. Using in vitro receptor autoradiography to rat brain sections [ 3H]Lu 28-179 binding was shown to be most predominant in the cerebral cortex, piriform cortex, hippocampal formation (CA1–CA3 and dentate gyrus), superior colliculus, molecular layer of the cerebellar cortex and locus coeruleus. Overall, these data are consistent with [ 3H]Lu 28-179 labelling a σ 2-like binding site.