To evaluate nitric oxide (NO) production and the energy state of the heart after endotoxin administration, Wistar rats were injected i.p. with 10 mg/kg Escherichia coli lipopolysaccharide (endotoxin). Morphologic changes, plasma nitrite concentration, expression of inducible NO synthase (iNOS), and cardiac energy state, as reflected by several metabolites, were observed chronologically 0 (control), 4, 6 and 8 h after endotoxin administration. Electrocardiography (ECG) demonstrated arrhythmia after endotoxin administration. Biochemically, NO production increased in blood and iNOS increased in the heart. The amount of myocardial beta-ATP measured by 31P magnetic resonance spectroscopy (31P-MRS) increased transiently and then decreased. This transient increase might be a hyperdynamic response to endotoxin administration. At 4 and 6 h after endotoxin administration, pH measured by 31P-MRS was slightly decreased, but this decline was not statistically significant. On the other hand, the amount of lactate in heart samples increased in the 1H magnetic resonance spectra (1H-MRS). Ultrastructurally, in cardiovascular tissue, intracytoplasmic organelles were observed to be injured in blood vessels and cardiomyocytes associated with mast cell infiltration. These results suggest significant metabolic and morphologic abnormalities in the heart after endotoxin administration.