The peripheral-type benzodiazepine receptor (PBR) is not only widely expressed throughout the body, but it is also genetically conserved from bacteria to humans. Many functions have been attributed to it, but its primary role remains a puzzle. In the current study, we stably transfected cultures of MA-10 Leydig cells with either control or 18-kDa PBR antisense knockout plasmids. The antisense knockout vector was driven by the human enkephalin promoter, which contains two cAMP response elements, such that cAMP treatment of transfected cells could superinduce 18-kDa PBR antisense RNA transcription and, hence, down-regulate endogenous 18-kDa PBR mRNA levels. Control and knockout MA-10 cell lines were then compared at the level of receptor binding, thymidine incorporation, and steroid biosynthesis. Eighteen-kilodalton PBR knockout reduced the maximal binding capacity of tritium-labeled PBR ligands, and the affinity of receptors to the ligands remained unaltered. Additionally, 24-h accumulation of progesterone was lower in the knockout cells. Exposure of the two cell types to 8-bromo-cAMP resulted in a robust increase in steroid production. However, a complex pattern of steroid accumulation was observed, in which further progestin metabolism was indicated. The later decline in accumulated progesterone as well as the synthesis of androstenedione were different in the two cell types. At the level of cell proliferation, reduction of 18-kDa PBR mRNA showed no effect. Thus, we conclude that the 18-kDa PBR may have a more important role in steroidogenesis than in proliferation in this Leydig cell line.