Diabetes which occurs spontaneously in the BB Wistar rat is associated with reduced fertility, predominantly in breeding males. In the first month of diabetes, there is a significant (p less than 0.05) reduction in serum testosterone associated with a transient decrease of serum LH and the accumulation of lipid in Leydig cells. Between one and three months of diabetes, there is an increase in both serum testosterone and LH and a further deposition of lipid droplets in Leydig cells. From three to six months of diabetes, there is a reduction of serum testosterone similar to age-matched controls, but high serum LH levels persist. Similar levels of LH and testosterone are noted after six months of diabetes, and all BB rats show marked changes in seminiferous tubules. These morphological changes in tubules consist of increased tubular wall thickness, severe germ-cell depletion, and Sertoli-cell vacuolization. Similar morphological changes of testes associated with generalized atrophy are noted in all control rats after 16 months of age. Decreased fertility in the BB rat appears to be associated with a primary disorder of Leydig cells, which precedes changes in seminiferous tubules consistent with accelerated aging. Preliminary data in impotent diabetic men suggest that the BB rat may be a valuable model for investigating human diabetic impotence and infertility.