Aims/hypothesis. Although matrix metalloproteinase-9 (MMP-9) is specifically induced and apoptosis of endothelial cells is evidenced in diabetes mellitus, the mechanism of endocardial endothelial dysfunction in diabetes mellitus is not clear. The increase in MMP-9 activity is associated with endocardial endothelial apoptosis and dysfunction in diabetes mellitus. Methods. Diabetes was created by injecting 65 mg/kg alloxan in tail vein of MMP-9 knockout (-/-) and wild-type (WT, C57BL/J6) mice. At 8 weeks mice were grouped: (i) WT+saline; (ii) WT+alloxan; (iii) MMP+saline; (iv) MMP+alloxan. The MMP-9 genotype was determined by observing single PCR product of different mobility than the PCR product from wildtype in blood from tail vein. Results. MMP-9 activity, measured by zymography, increased in plasma and in the left ventricle of alloxan-induced diabetic wild-type mice. The concentrations of cardiac inhibitor of metalloproteinase, that blocks MMP-9 activity, were decreased in diabetic MMP-9 knockouts as well as in wild-type mice. Diabetes induced apoptosis, detected by TUNEL assays, in wild-type but not in MMP-9 knockouts. Endocardial endothelial function was severely impaired in diabetic wild-type mice compared with normoglycaemic animals, while non-diabetic MMP-9 knockout mice showed partial endocardial endothelial dysfunction which was not further exacerbated by the developments of diabetes. Conclusion/interpretation. The results suggest an association between increased MMP-9 activity and endocardial endothelial apoptosis in diabetic mice, while genetic ablation of MMP-9 correlated with amelioration of endocardial endothelial dysfunction and apoptosis. [Diabetologia (2003[Diabetologia ( ) 46:1438[Diabetologia ( -1445 Keywords Fibrosis, TUNEL, endocardial endothelium, MMP, TIMP, hypertension, heart failure. Corresponding author: Dr. S. C. Tyagi, Department of Physiology and Biophysics, University of Louisville, 500 South Preston Street, Louisville, KY 40292 USA E-mail: s0tyag01@louisville.edu Abbreviations: A, Alloxan; CIMP, cardiac inhibitor of metalloproteinase; ECM, extracellular matrix; EE, endocardial endothelial; eNOS, endothelial nitric oxide synthase; LV, left ventricle; MMP, matrix metalloproteinase; MUP, mouse urinary protein; NO, nitric oxide; PCR, polymerase chain reaction; TIMP, tissue inhibitor of metalloproteinase; TUNEL, terminal uridine deoxynucleotide neck-end labeling; WT, wild-type.