Specific binding of [125I]iodomelatonin to homogenates from purified rat liver nuclei was characterized. The binding is rapid, reversible, saturable and of high affinity. Specific binding seems to be found in the nuclear protein fraction, since after precipitation of the proteins with trichloroacetic acid, the specific binding disappeared. The Kd (180 +/- 20 pM) and Bmax (9.1 +/- 0.03 fmol/mg protein) values (mean +/- S.E.M.) agree with the melatonin concentration in nuclei and may imply a physiological locus for melatonin action.