The effects of Cd administered ip in a single carcinogenic dose (1 mg Cd/kg body weight) into male CFLP mice were investigated on the histological and cytogenetic status of the testes of the animals on d 3 and 6 mo after the Cd injection. There were some characteristic histological changes in the testes of the mice on d 3 after Cd treatment: the presence of interstitial hemorrhages, the death of Leydig cells, sterile necroses of tubular elements, and numerous atypical mitoses of the spermatocytes. A significant increase compared with the control was found only in the number of numerical chromosome aberrations in the Cd-treated group. Six months later, a significant difference compared with the control was found only in the relative weight of the testes in the treated group. There were soon no detectable effects of Cd on the histological picture of the testes and on the spermatocytes.