Since lithium has been shown to inhibit release of iodine from the thyroid, we have investigated its therapeutic potential in thyrotoxicosis. Eight detailed (131)I kinetic studies were performed on seven thyrotoxic women and data was analyzed using a computer program. Lithium at serum levels of about 1 mEq liter decreased the loss of (131)I from the thyroid, led to a fall in serum (131)I levels and diminished urinary (131)I excretion. Computer simulation of the lithium effect required, in every case, that lithium inhibit hormonal and nonhormonal thyroid iodine release. In five cases a second lithium effect was required for a satisfactory fit of the model soluton with observed data: namely, an inhibition of hormone disappearance from serum. NEITHER INHIBITION OF RELEASE NOR OF HORMONE DISAPPEARANCE SEEMED TO BE AFFECTED BY METHIMAZOLE (RELEASE: 52% decrease without methimazole, 60% with methimazole; hormone disappearance: approximately 60% decrease in both). When Li(+) was discontinued, recovery of the iodine release rate and hormone disappearance rate over the observed time span was variable, ranging from no recovery to rates that exceeded pre-Li(+) values. When Li(+) is used alone its effect on serum hormone levels is diminished due to continued accumulation of iodide by the thyroid. Thus, serum thyroxine-iodine levels fell 21-30% in 6-8 days in patients who did not receive methimazole and 15-67% in the methimazole-treated subjects. For prolonged therapy, therefore, a thiocarbamide drug must be used in conjunction with Li(+). The similarity of inhibition of iodine release from the thyroid produced by Li(+) and iodides is discussed.