Nickel subsulfide (Ni3S2) was injected in various amounts into the testis of adult Fischer rats for the study of the acute and chronic effects of Ni3S2 on testicular cells. Rats given injections of 0.6 to 10 mg of Ni3S2 developed an immediate inflammatory response at the site of injection, followed by a delayed, slowly evolving coagulation necrosis of seminiferous tubules and interstitial cells. The extent of testicular necrosis was dose dependent, but at doses of 5 or 10 mg of Ni3S2 the rats invariably developed subtotal destruction of the testis. The testis became atrophic, without regeneration of seminiferous tubules. No damage was seen in the other testis, and no systemic effects were noted. Malignant testicular neoplasms developed in 16 of 19 rats within 20 months after an injection of 10 mg of Ni3S2. These neoplasms were classified by light and electron microscopy as fibrosarcomas, malignant fibrous histiocytomas, and rhabdomyosarcomas. None of the testicular neoplasms was derived from germ cells or genital cord cells. The occurrence of rhabdomyosarcomas in the testis, an organ normally devoid of striated muscle, suggests that Ni3S2 induces malignant transformation of undifferentiated, pluripotential mesenchymal cells.