Internal mammary artery specimens from 17 patients were each divided into three separate rings. One ring (control) remained in Krebs solution and the other two were clamped for 30 minutes with either a soft or hard jaw clamp. Isometric tensions were measured in an organ chamber by contracting the rings twice with a thromboxane A2 mimetic, U46619, and relaxing the rings first with the endothelium-dependent agent acetylcholine followed by the endothelium-independent agent sodium nitroprusside. Endothelium-dependent maximal relaxation of the rings was impaired from control after both soft (20% versus 91%; p < 0.01) and hard (1% versus 91%; p < 0.01) jaw clamps were used. However, relaxation after use of hard jaw clamps was significantly less than after use of soft jaw clamps (1% versus 20%; p < 0.05). Endothelium- independent maximal relaxation was not impaired from control after soft jaw clamps (89% versus 97%) were applied but was significantly impaired after use of the hard jaw clamps compared with control (73% versus 97%; p < 0.01) and compared with soft jaw clamps (73% versus 89%; p < 0.05). Rings of internal mammary artery specimens from 10 patients from each experimental group were silver stained. The percentage of intact endothelial cells was significantly greater after soft jaw clamping than after hard jaw clamping (39% versus 15%; p < 0.02). These data suggest that soft jaw clamps significantly reduce the degree of vasoactive dysfunction compared with hard jaw clamps. In addition, soft jaw clamps produce fewer morphologic changes in the human mammary artery after temporary occlusion.