By 3 months after surgery, testicular weights were 12-14% below control values in a large series of vasectomized rats of the inbred Lewis strain. Testicular sperm concentrations in the vasectomized rats were 25-29% below those in the sham-operated animals. Although modest in magnitude, these differences were highly significant (P less than 0.001). Histometric analysis of testes from a smaller series of vasectomized Lewis rats revealed various proportions of abnormal seminiferous tubules. The degenerative changes included reduction in tubule diameter, fusion of spermatids, and depletion of advanced germinal elements. In the most severely affected tubules, leakiness of the blood-testis barrier was indicated by penetration of lanthanum through Sertoli-cell tight junctions.