The pharmacokinetics of methimazole (MMI) administered intravenously and orally were determined in six adult domestic shorthaired cats. There was no significant difference between mean serum MMI concentrations after oral and i.v. administration by 30 min post-MMI administration, indicating relatively rapid and complete absorption of the drug. The bioavailability of MMI ranged from 27% to 100% (mean=81.1±11.4%). The mean serum elimination half-life was 6.6±2.0 h, with a wide range of values (1.9h to 15.1h). After repeat i.v. administration of MMI following 2 weeks of oral administration of the drug, no significant difference was found between mean serum concentrations after single-dose and multiple-dose administration. No significant change in serum elimination half-life or total body clearance was found after multiple-dose administration of MMI. Two cats with the longest half-lives (9.9h and 15.1h), however, did exhibit markedly shorter t1/2, values (3.5h and 3.3h, respectively) after multiple-dose administration. Values for central and steady state volumes of distribution also decreased after multiple-dose administration, possibly indicating saturation of thyroid uptake of MMI with chronic administration. These results indicate that MMI has good oral bioavailability and has a longer mean serum elimination half-life than propylthiouracil, the other anti-thyroid drug that has been evaluated in cats. Although no significant change in mean values occurred after multiple-dose administration of MMI, drug-induced acceleration of metabolism may occur in some cats after long-term MMI administration.