Effect of orchiectomy on peripheral benzodiazepine receptors (PBZr) density in hormone sensitive, poorly differentiated Dunning G rat prostatic tumors were studied. A significant increase in PBZr density (1.4-fold) was observed in both mitochondrial fractions (m-fractions) and microsomal fractions (p-fractions) of tumors from orchiectomized rats as compared to tumors from sham operated controls. Substitution with testosterone in orchiectomized rats repressed the PBZr density in both m- and p-fractions to that of the control animals. The data strongly suggest a regulatory role of testosterone on the PBZr density in these hormone sensitive prostatic tumors.