Endothelin ET-1 plays an important, however not entirely identified role in physiology and pathology of thyroid gland. The aim of the study was to determine if in patients with non-toxic nodular goiter there is relationship between size of goiter and ET-1 concentration in plasma of peripheral venous blood and plasma of venous blood taken from thyroid vein in patients undergoing strumectomy. The study included 30 patients with non-toxic nodular goiter (29 women and 1 man, mean age 48 year) and 15 healthy persons of control group. In all ET-1 concentration in plasma of venous peripheral blood and T3, T4, TSH serum concentration were estimated using radioimmunologic methods, as well as size of thyroid gland were evaluated in ultrasonography. Patients with nodular goiter underwent strumectomy. During the operation, before strumectomy, blood from thyroid vein was sampled to estimate plasma ET-1 concentration. Patients were divided into two groups: with adenomatous goiter (14 patients) and colloid goiter (16 patients). The goiter size was similar in each group. In all of the groups there was no relationship between goiter size and ET-1 concentration in plasma of peripheral and thyroid vein. Lack of correlation between ET-1 concentration in plasma and goiter size does not exclude possible ET-1 role in goitrogenesis, it suggests that ET-1 may play a role in control of thyroid gland trophic regulation and vascularity as paracrine or autocrine factor. Significantly higher concentration of ET-1 in plasma of thyroid vein in comparison to plasma of peripheral venous blood and positive correlation between ET-1 concentration in thyroid and peripheral plasma were found in both studied groups of patients with goiter and this may speak for increased ET-1 production within the goiter.