The formaldehyde histofluorescence method and receptor autoradiography were respectively used to investigate the distribution and densities of the catecholamine (CA) terminals and [3H]-etorphine binding sites, after 20 days of injection of 6-hydroxydopamine (6-OHDA) into N. subcoerulens of the rabbit. The results showed that both CA terminals and [3H]-etophine binding sites in the preoptic area, especially in the medial preoptic area, were reduced in 6-OHDA pretreated animals, thus suggesting the possible location of opioid receptors on the CA terminals.