Monoamine oxidase (MAO) activity and its forms were measured in normal human thyroid glands (39 samples from necropsy and four from perinodular tissues) and in pathological thyroid glands (13 multinodular goitres, three carcinomas, seven chronic thyroiditis, five Graves' disease and seven follicular, seven foetal and five embryonal adenomas). MAO activity in carcinomas was lower than in normal perinodular thyroid control tissue; in Graves' disease, foetal and embryonal adenomas it was significantly higher than in the normal control. The use of clorgyline (a selective MAO A inhibitor) and deprenyl (selective MAO B inhibitor) allowed estimation of the relative proportion of MAO forms. A high proportion of MAO A (more than 90%) was observed, both in normal and in pathological thyroid tissues. Changes in total thyroid MAO activity, but not in its forms, were found in different pathologies of the thyroid gland.