The authors compared the size of the thyroid gland , asessed by ultrasonographic examination of its volume and the functional state of the thyroid by examination of the serum level of TSH and anti-TPO antibodies in 492 pupils aged 10 and 13 years in two close agricultural areas. The areas differed above all as to the nitrate content of drinking water. In communities where the source of drinking water were private wells (nitrate area), as much as 68.1 % of the water samples had a nitrate content > 50 mg/l and 46.4 % samples > 100 mg/l. In communities where the source of drinking water was a water main with a known source (control area) the water samples did not exceed 50 mg/l (73.7 % > 15 mg/l and 26.3 % > 50 mg/l. Results:pupils from the nitrate area had a larger thyroid gland similarly as older pupils from the control area. A volume above 7 ml/m 2 in the nitrate area was recorded in the group of 10-year olds in 25/99 (27.7 %) and in the group of 13-year-old ones in 33/154 (21.4 %), while in the control area it was in the 10-year-old ones 11/92 (11.9 %) and in the 13-year-old ones 16/156 (10.2 %, P < 0.01. In the nitrate area TSH > 5 mlU/l was recorded in 5.6 % and higher anti-TPO antibodies in 2.6 % pupils. In the control area the corresponding figure was 1.1 % pupils, P < 0.01.

        Key words: Nitrates - Drinking water - Thyroid gland