The iodide concentrating mechanism, measured as the gland to plasma (G/P) 125I- concentration ratio, was measured in male mouse submandibular glands after the animals had been fed with varying amounts of lithium chloride for intervals ranging from 0 to 8 weeks. After acute exposure (by injection) to lithium the G/P 125I- fell compared to control values and was further depressed at high serum lithium values. Chronic exposure to lithium was characterised by an increase in the G/P 125I- ratios at therapeutic serum lithium concentrations. It is suggested that lithium has a direct effect on the iodide concentrating mechanism that may be independent of any action of thyroid stimulating hormone during this process. The effect may be important in the formation of lithium induced goitre.