Abstract Groups of rats were fed maneb at dietary levels from 250 to 1500 ppm or ethylenethiourea (ETU) from 5 to 500 ppm for 60 days. After this time, both compounds had induced similar changes in the electrophoretic pattern of thyroid-tissue extracts. These changes were a decrease in the density of a band presumably representing thyroglobulin, an increase in the density of a band representing a non-iodinated protein, and a slight decrease in the density of a band representing an iodinated protein. After electrophoresis of the thyroid extracts from ETU-fed rats given 131I 24 hr before sacrifice, three iodoprotein groups were found. The decrease in radioactivity in the thyroglobulin band with increasing dose of ETU paralleled the change in density of the band.