AbstractA procedure has been developed for the immunohistochemical localization of morphine in the rat basal ganglia. Corpora striata of Sprague-Dawley rats, treated with morphine sulfate subcutaneously, were removed and quick frozen in isopentane/liquid nitrogen. Fiveμm sections were reacted with various dilutions of goat anti-morphine serum (IgG), rabbit anti-sheep and rabbit anti-goat peroxidase-labeled IgG, diaminobenzidine and counterstained with methyl green. Light microscopic presence of dark brown reaction product indicated positive staining. Diffuse and particulate cytoplasmic and plasmalemmal staining were observed. Nonspecific background staining was substantially eliminated by adsorbing the morphine antiserum with lyophilized rat brain followed by DEAE-cellulose chromatography prior to the immune reactions. The technique utilizes a two-stage conjugation reaction employing peroxidaselabeled immunoglobulin.