The purpose of this study was to investigate the role of osteopontin (OPN) in diabetic heart. Diabetes was induced in wild-type (WT) and OPN knockout (KO) mice using streptozotocin (150 mg/kg) injection. Left ventricular (LV) structural and functional remodeling was studied 30 and 60 days after induction of diabetes. Induction of diabetes increased OPN expression in cardiac myocytes. Heart weight/body weight ratio was increased in both diabetic (D) groups. Lung wet weight/dry weight ratio was increased only in WT-D group. Peak LV developed pressures measured using Langendorff perfusion analyses were reduced to a greater extent in WT-D versus KO-D group. LV end-diastolic pressure-volume curve exhibited a significant leftward shift in WT-D, not in KO-D group. LV end diastolic diameter, percent fractional shortening and the ratio of peak velocity of early and late filling (E/A wave) were significantly reduced in WT-D mice as analyzed by echocardiography. The increase in cardiac myocyte apoptosis and fibrosis was significantly higher in WT-D group. Expression of atrial natriuretic peptide and transforming growth factor-1 was significantly increased in WT-D group. Induction of diabetes increased protein kinase C (PKC) phosphorylation in both groups. However, phosphorylation of PKC-II was significantly higher in WT-D group, while phosphorylation of PKC- was significantly higher in KO-D. Levels of peroxisome proliferator-activated receptor- were significantly decreased in WT-D, not in KO-D group. Thus increased expression of OPN may play a deleterious role during streptozotocin-induced diabetic cardiomyopathy with effects on cardiac fibrosis, hypertrophy and myocyte apoptosis.