1. Heme compounds, like hemin, a heme oxygenase-1 inducer, are used in the treatment of acute porphyria treatment. The side-effects of hemin on haemostasis have been reported. To address those effects, in the present study we used a sensitive, high-frequency ultrasound technique to record acoustic velocity and to investigate whole blood clotting in Wistar rats treated chronically with hemin (50 mg/kg per day). 2. The hemin-induced disturbances in haemostasis measured were comparable to the heparin reference treatment, with a significant decrease in clotting velocity in both groups compared with controls (e.g. the time to clot was 40 +/- 5, 53 +/- 13 and 10 +/- 2 min, respectively; P < 0.05). Precautions must be taken when using high doses of hemin or in the treatment of bleeding diseases. 3. Further investigations are required to explore the effects of hemin in thrombosis models, because it could be a promising 'old drug' for the treatment of venous thrombosis in patients.