Hyperplasia, and loss of colloid in the thyroid glands were observed in 16 white rats which had received 0.05 to 0.1% bis-4-acetamino-phenyl-selenium di-hydroxide in their diet for 10 days. Eight rats which received 0.05% of the compound for 105 days had multiple adenomas of the thyroid, and adenomatous hyperplasia of the liver.