Laser-induced thrombi have been frequently produced in rat mesenteric vessels to investigate the effect of antithrombotic drugs. We have tested low molecular heparins in comparison to unfractionated heparin. The investigations were carried out on male Wistar rats weighing 200-300 g; the animals were anesthetized with pentobarbital sodium (Nembutal) 60 mg/kg i.p. Vascular lesions were induced with a Coherent CR-2 supergraphite ion laser (argon laser) mounted on a Leitz Orthoplan microscope. An intestinal loop was spread on a self-constructed object stage, mounted on the microscope table. The laser beam was directed through the optical path of the microscope on small mesenteric vessels in the fat-free portion of the mesentery. For the evaluation of thrombus formation the number of laser injuries needed to induce a defined thrombus was used. All low-molecular-weight heparins (preparation B, Braun; BR-Z-0601, Braun; CY 216, Choay; Fragmin, Kabi; low molecular heparin, Sandoz and Org 10172, Organon) or unfractionated heparin (Liquemin; Hoffmann-La Roche) showed a significant and dose-dependent antithrombotic effect after subcutaneous injection, if venules had been damaged. The effect lasted for 48 h, partly for over 48 h. If thrombus formation was studied after intravenous injection into arterioles as well as into venules 8 h after injection, no antithrombotic effect could be demonstrated under the influence of CY 216 or unfractionated heparin.