Little is known about the toxic activity of the atrazine (a herbicide, commonly used in agricultural production) on the thyroid gland. In this study the compound was administered orally in female albino rats at sublethal exposure equivalent to 0.2 LD50 doses for 6 and 12 days. At termination of dosing the anesthetized animals were killed and blood was drawn for the determination of serum triiodothyronine (T3) and thyroxin (T4). A dose-dependent decrease of serum T3 concentration was observed in all the groups (control: 0.57 nmol·L-1; 6 days: 0.35 nmol·L-1; 12 days: 0.21 nmol·L-1). The thyroid gland was examined light-microscopically. Bouin’s solution-fixed thyroids were embedded in paraffin and sections cut at 6 μm, stained separately with toluidine blue according to Slinchenko’s method. Histologically in experimental groups epithelium featured small cuboidal cells and occasional structures of the follicles confluence within epitheliomers. A dose-dependent changes of the following parameters were observed: (a) increasing of number of follicle-building thyroid cells; (b) increasing of follicular volume, (c) decreasing of nucleus volume. Investigation of the whole population of thyroid mast cells disclosed no change in degranulation intensity. By contrast, degranulation intensity was decreased in perifollicular mast cells from groups treated with atrazine in dose-dependent manner.There are no changes observed in degranulation of stromal mast cells. These results suggesting that differences in response to the atrazine might account for an aspect of the functional heterogeneity within the rat thyroid mast cell population. Little is known about the toxic activity of therazine (a herbicide, commonly used in agricultural production) on the thyroid gland. In this study the compound was administered orally in female albino rats at sublethal exposure equivalent to 0.2 LD50 doses for 6 and 12 days . At termination of dosing the anesthetized animals were killed and blood was drawn for the determination of serum triiodothyronine (T3) and thyroxin (T4). A dose-dependent decrease of serum T3 concentration was observed in all the groups (control: 0.57 nmol · The thyroid gland was examined light-microscopically. Bouin’s solution-fixed thyroids were embedded in paraffin and sections cut at 6 μm, L-1, 6 days: 0.35 nmol·L-1, 12 days: 0.21 nmol·L- stained separately with toluidine blue according to Slinchenko’s method. Histologically in experimental groups epithelium featured small cuboidal cells and occasional structures of the follicles confluence within epitheliomers. A dose-dependent changes of the following parameters of observed: (a) increasing of number of follicle-building thyroid cells; (b) increasing of follicular volume, (c) decreasing of nucleus volume. Investigation of the whole population of thyroid mast cells showing no change in degranulation intensity. By contrast, degranulation intensity was decreased in periollicular mast cells from groups treated with atrazine in dose-dependent manner. Here are no changes observed in degranulation of stromal mast cells. These results suggest that differences in response to the atrazine might account for an aspect of the functional heterogeneity within the rat thyroid mast cell population.