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Male Wistar rats were given 0-200 mg/l of nabam and 0-300 mg/l of ethylenethiourea (ETU) in drinking water ad libitum for 28 d to study their effect on thyroid gland function and morphology. Blood samples for RIA hormone assays (T3, T4, TSH) were taken and thyroids of each rat were dissected out for routine light and electron microscopic analysis after sacrifice performed under CO2 anesthesia. Nabam (8.4-30.5 mg/kg bw/d) inhibited dose-dependently T4-secretion and increased T3-secretion, but had no effect on basal TSH-secretion. ETU (10.6-23.4 mg/kg bw/d) inhibited dose-dependently T4- and T3-secretion. ETU also increased basal TSH-secretion tenfold, obviously due to the lack of negative feedback by T4 and T3. Light microscopic analysis revealed neither nabam nor ETU induced morphological alterations in thyroids. Both but especially ETU induced, ultrastructural changes, namely increased number of myelin bodies, dilatation of the rough endoplasmic reticulum, and increased vacuolization in the epithelial cells of thyroid follicles. The functional alterations are obviously connected with, and preceed slight ultrastructural degenerative changes in thyroid follicle epithelial cells even at low doses of nabam, and ETU, after a short-term oral administration.