The visualization of opiate binding sites within the hippocampus of the rat has been achieved by means of an in vitro autoradiography. In line with the concept of multiple opiate receptors, different opioid agonists revealed a particular distribution pattern. Whereas the selective delta-receptor agonist [3H]D-Ala2,D-Leu5-enkephalin specifically labelled binding sites in the CA2 area, [3H]etorphine grains displayed a uniform dense distribution throughout the pyramidal cell layers from CA1 to CA4.