Peripheral benzodiazepine receptors (PBR) have been localized to the outer mitochondrial membrane in a variety of organs, where they apparently play a role in steroidogenesis, oxidative processes, and/or growth and development. Previous studies have demonstrated ontogenetic changes in heart and lung PBR, with maximal PBR density at 31 days, as opposed to negligible changes in brain PBR during the prenatal through postnatal periods. The present study was designed to examine the influence of maturation and aging upon PBR binding characteristics. Rats aged 1, 2, 12, 18, and 24 months were sacrificed, and the following organs were removed according to standard protocol: heart, lungs, kidneys, adrenal gland, and testes. Binding studies were performed using [3H]PK 11195 as a radioligand. A 3-fold increase in PBR density was demonstrated in testis during maturation, with maximal values appearing at 18 months, followed by a decline at 24 months. None of the other organs examined showed significant changes in PBR density. No alterations were observed in affinity values for the various organs and ages. Since testicular PBR are putatively involved in testosterone production, these results might reflect critical interactions between PBR and gonadal hormone activity during development.