The wealth of knowledge about the function and regulation of adult Leydig cells, the cells within the mammalian testis that produce testosterone, make these cells ideal for studying principles and mechanisms of aging. A hallmark of mammalian aging is decreased serum testosterone concentration. In the Brown Norway rat, this has been shown to be associated with the reduced ability of aged Leydig cells to produce testosterone in response to LH (LH). Herein we demonstrate that culturing the aged cells with dibutyryl cAMP (dbcAMP), a membrane-permeable cAMP agonist that by-passes the LH receptor-adenlyly cyclase cascade, restores testosterone production to levels comparable to those of young cells, and also restores steroidogenic acute regulatory protein (StAR) and P450scc, the proteins involved in the rate-limiting steps of steroidogenesis. These results strongly suggest that signal transduction deficits are responsible for reduced steroidogenesis by aged Leydig cells, and that by-passing signal transduction reverses the steroidogenic decline by the aged cells.