Considerable evidence exists to indicate that the Leydig cells influence the seminiferous tubule by maintaining a high concentration of testosterone in the peritubular compartment of the testes. Recent studies using various types of agents to disrupt spermatogenesis, have shown that significant changes occur in the structure and function of the Leydig cells. In these situations, Leydig cells enlarge, show hyperresponsiveness to hCG stimulation in vitro and contain a reduced number of receptors for LH/hCG. Evidence is presented to support the hypothesis that the changes in Leydig cell structure and function are the result of local factors. The data provide support for the concept that the functioning of Leydig cells is modulated by the seminiferous tubules.