To study the effects of seminiferous tubule damage on Leydig cell function and morphology, rats were treated by fetal irradiation (to induce Sertoli cell-only syndrome, SCO), 3 months administration of hydroxyurea (HU), or chronic feeding of a vitamin A-deficient diet (VAD). Leydig cell function was assessed by the measurement of serum LH and testosterone and the response of serum testosterone to hCG stimulation, while morphology was studied by electron microscopy after perfusion fixation. Serum LH was significantly elevated in each experimental group, while basal serum testosterone was significantly lower only in SCO rats. In all treatment groups, the serum testosterone response to hCG was significantly decreased when measureed as the area under the response curve. Despite a decreased response to hCG, the Leydig cells were larger than normal and showed striking increases in quantities of smooth endoplasmic reticulum, mitochondria and Golgi complex. Leydig cell dysfunction has been demonstrated in animals with varying degrees of seminiferous tubule damage, but paradoxically the cytological features of the Leydig cells were indicative of hypertrophy.