Eighteen nodules from thirteen patients with non-toxic nodular goitre were studied. The nodules were classified, according to the 131I scintiscanner, into »cold« and »warm« types. The histological diagnosis of all the glands was multinodular colloid goitre. An enzymatic system with iodide-peroxidase activity was prepared from nodular tissue obtained by surgical thyroidectomy. The enzymatic activity was determined by spectrophotometry at 287.5 nm by measuring the formation of triiodide ion. The group of »cold« nodules showed an average of 333 units of enzymatic activity and the »warm« nodules 940 units. The significance of the correlation between the capacity for iodide-uptake, the iodide-peroxidase activity and the involution of the metabolic steps in the nodular goitre is discussed.