The effect of chronic suramin administration on rat testis was evaluated. The animals were given the drug twice a week for two months, following a treatment schedule comparable to that used in cancer therapy. Light microscopic sections revealed vacuolizations near the basal lamina, germ cells very little differentiated, together with immature cells located in the lumen of seminiferous tubules. Ultrastructural changes were revealed in the form of a decrease in nuclear density with evaginations of the nuclear envelope and sometimes ruptures of the plasmatic membrane. Changes in the Sertoli cells consisted of cytoplasm vacuolizations, in which some lysosomes, containing degraded material such as myelin-like figures, were present.