For many years there has been speculation about possible damage to the salivary glands following administration of ablative doses of radioactive iodine for treatment of hyperthyroidism. We have investigated the changes that occur in the composition of saliva in hyperthyroidism and after the administration of an ablative dose of radioactive iodine to hyperthyroid subjects. The study consisted of two parts: first, a comparison of a group of hyperthyroid patients with a group of normal subjects with regard to the concentration or activity of 10 constituents of saliva; second, measurement of those constituents 3-42 weeks after administration of 370 MBq of radioactive iodine to a group of hyperthyroid subjects. Saliva specimens from 38 untreated out-patients with hyperthyroidism due to Graves' disease or toxic nodular goitre were studied to evaluate the effects of hyperthyroidism and the results were compared with a group of 93 normal subjects. Seventy-one samples of saliva from 26 patients with persistent hyperthyroidism were collected and analysed 3-42 weeks after radioactive iodine administration. The flow rate; the concentrations of total protein, iodine, calcium, urate, phosphate, potassium and immunoglobulin A; and the activities of N-acetylglucosaminidase, lysozyme and lactate dehydrogenase were measured. In hyperthyroidism the salivary flow rate and the concentrations of urate and potassium were significantly (P < 0.05) increased and the concentrations of total protein, calcium and lactate dehydrogenase activity significantly decreased compared to the control group. After radioactive iodine was administered, significant positive trends were observed in the concentrations of total protein, N-acetylglucosaminidase and immunoglobulin A. These trends were independent of the free T3 levels obtained from the same specimens. Hyperthyroidism leads to a number of changes in salivary composition. For most of the salivary components measured no significant changes were observed 3-42 weeks after administration of 370 MBq of radioactive iodine to patients with persistent hyperthyroidism. The relatively small positive trends in the concentrations of total protein, N-acetylglucosamidase activity and immunoglobulin A may have been due either to changes in thyroid status or to the effects of radiation on the salivary glands, or both.