After a single oral dose of 25 mg/kg tranexamic acid in 37 patients with cataract as the sole eye disease, the concentration of the drug was measured in both the serum and the aqueous at various intervals following intake. The serum concentration was highest after three h (average 15.44 mg/l), but a trace of tranexamic acid (0.7 mg/l) could still be found after 19 h. In the aqueous, the concentration was likewise highest after three h (average 1.62 mg/l), but, following this, the fall in tranexamic acid concentration was very gradual and after 19 h was found to be 1.3 mg/l. In two patients who had received 25 mg/kg three times daily for three days, an aqueous concentration of 2.3 mg/l was found eight h after the final intake.