The goitrogenic action of PAS was discovered in 1951. Previous investigators have interpreted this side-effect of PAS as an influence on the iodine turnover similar to that of thiouracil and related substances. Investigations by means of radioiodine in a number of patients with goitre developed during PAS-treatment have revealed, however, that the results differ from the values found in studies of patients who were treated with thiouracil-like compounds. The aim of the present study has been to elucidate the influence of para-aminosalicylic acid on the iodine metabolism by a systematic It3l investigation in a group of PAS-patients. The study incorporates a summary of the normal iodine metabolism in man and a survey of the known congenital or iatrogenic changes in the iodine metabolism. As a basis for an evaluation of the tracer data from the PAS-patients, results of similar studies in patients with known thyroid disorders are included. These investigations were carried out during the same period and with the same technique as that applied in the PAS-patients.