The highest maximum binding capacities at the mu-sites of rabbit brain are in the striatum, with intermediate levels in the diencephalon, mesencephalon, cerebellum and cortex and low levels in the pons-medulla and hippocampus. For the delta-site the highest maximum binding capacity is also in the striatum; there are almost equally low levels in the other brain regions. At the kappa-sites the maximum binding capacities are highest in the diencephalon; there are intermediate levels in the cortex and striatum, and low levels in the mesencephalon, cerebellum, hippocampus and pons-medulla. The KD values lack reproducibility; there are no regional variations at the kappa-site as estimated with [3H](-)-bremazocine, but the possibility cannot be excluded that there are regional variations in the KD values for [3H][D-Ala2,MePhe4,Gly-ol5]enkephalin at the mu-site or for [3H][D-Ala2,D-Leu5]enkephalin at the delta-site. It may be important to use saturation analysis in future investigations of the distributions of the binding sites.