1. Thrombin caused a tonic contractile response in rabbit aortic strips which showed tachyphylaxis. 2. Thrombin-induced contraction was partially dependent upon extracellular calcium. 3. Contractile response by lower concentrations of thrombin was suppressed by the endothelium. This endothelial effect was blocked by methylene blue, hemoglobin, bromophenacyl bromide or removal of extracellular calcium but not by indomethacin, nordihydroguaiaretic acid or nifedipine. 4. Cyclic GMP levels were not different between the thrombin-stimulated and control strips. 5. Thrombin could not stimulate prostacyclin release from the aortic strips. 6. These results suggest that thrombin possesses a contractile action in rabbit aortic smooth muscle which is attenuated by endothelium-derived relaxing factor (EDRF) spontaneously released from the endothelium during the contraction.