Under the conditions of these 2-year inhalation studies, there was clear evidence of carcinogenic activity of nickel subsulfide in male F344/N rats based on increased incidences of alveolar/bronchiolar adenoma, carcinoma, and adenoma or carcinoma (combined) and on increased incidences of benign, malignant, and benign or malignant (combined) pheochromocytoma of the adrenal medulla.