Serum levels of diiodotyrosine (DIT) and urinary excretion rates of DIT and iodine were measured in 10 normal subjects after oral administration of 1.57 mumol of DIT corresponding to 400 micrograms of iodine. Serum DIT concentrations rose promptly from a mean endogenous basal level of 0.23 nmol/l to maximum values between 6.0 and 20 nmol/l within 30 min to 1 h after DIT ingestion. Decreasing DIT levels were found in all subjects 2 h after DIT intake. Urinary excretion of intact DIT was low, being less than 1% of the administered dose of exogenous DIT within 2 days. In contrast, 52% of the iodine administered in the form of DIT was excreted in the urine in the same time interval. The rapid absorption of DIT from the gastrointestinal tract combined with rapid and almost complete metabolic degradation by deiodination make orally applied DIT seem a suitable iodine carrier compound for therapeutic purposes.