1. The study aimed to investigate the pharmacokinetics of thrombomodulin alpha (TM-alpha), human-soluble thrombomodulin in rats. 2. Intravenously administered TM-alpha was eliminated in two phases (T(1/2 alpha) = 0.2-0.3 h and T(1/2 beta) = 6-8 h), and the elimination curve was linear in a dose range of 10-250 microg kg(-1). Based on the results of tissue concentration studies after reaching the steady-state, the highest concentration of TM-alpha was seen in the plasma, suggesting the low levels of transfer to tissues (< or = 22% of plasma levels). 3. In vivo metabolism of TM-alpha was also analysed using high-performance liquid chromatography. The main peak observed in the plasma was TM-alpha, and even 72 h after the last dose of repeated administrations, 80% or more was unchanged form. Approximately half of the radioactivity excreted in the urine was recovered as a peak corresponding to TM-alpha. 4. The results reveal that although plasma clearance was lower in the renally impaired rats, the decrease was not large, with a difference of only about 20%. As a result, although the cause remains unclear, it is considered unlikely that the plasma concentrations of TM-alpha will vary considerably in patients with renal impairment.