doses, and partly because of the biological variability in the radiosensitivity of different glands. With his method thyrotoxic patients can be treated safely and cffectively without elaborate techniques. Though we do not allow for the percentage uptake of 3 I by the gland in deciding on the dose to be given, we consider preliminary tracer investigations desirable to confirm the diagnosis. Pre-treatment of thyrotoxic cases with methylthiouracil and thyroxine before giving 1311 has been used by Fraser et al. (1954) to produce a more uniform uptake of the isotope by the gland. We investigated the effects of pre-treatment with methylthiouracil on the results of 131 therapy. A series of 28 cases treated with methylthiouracil until one week before the 13 11 was administered provided a significantly smaller number of one-dose cures (28%) than a comparable series which received no premedication (75%). Further investigations have shown that this effect is probably due to an increased degree of resistance to radiation conferred on the gland by the pre-treatment. This factor must be taken into account whatever scheme of dosage is used, and further observations suggest that a dose at least 25% larger must be given to those who have recently received methylthiouracil.