The disposition of vitamin K1, after intravenous (10 mg) and oral doses (10 mg and 50 mg) was studied in six healthy male subjects. After intravenous administration, the plasma concentration-time profile was adequately fitted with an average terminal half-life of 1.7 h. After oral administration (10 mg and 50 mg) the availability of vitamin K showed marked inter-individual variation (10-63%). With the higher dose intra-individual variation was also observed. Experiments in brodifacoum-anticoagulated rabbits demonstrate that the duration of action of a pharmacological dose (10 mg/kg) is short (9 h) and that high plasma concentrations (ca 1 microgram/ml) of the vitamin are required to drive clotting factor synthesis during maximum coumarin anticoagulation. Taken collectively, these data indicate that the short duration of action of vitamin K, frequently observed in cases of coumarin poisoning, is a consequence of requirements for high vitamin K concentrations and rapid clearance of the vitamin.