OBJECTIVE Bolus injection of magnetic resonance (MR) contrast media has been used in recent years to exploit the diagnostic advantage of newer fast MR imaging sequences. The bolus effects of three equimolar dosages of ionic and nonionic magnetic susceptibility contrast agents on several cardiovascular functional parameters are investigated in normal rats and in rats subjected to acute myocardial infarction. These results are related to the osmolalities of the injected solutions. METHODS Four groups of rats were examined (n = 10 rats per group). Twenty normal rats were studied. Acute myocardial infarction was produced by ligating the anterior branch of the left coronary artery for 2 hours in another 20 rats. Sequential equimolar doses of 0.1, 0.3, and 0.5 mmol/kg of ionic dysprosium diethylenetriamine pentaacetic acid dimeglumine ([NMG]2DyDTPA) or nonionic dysporosium diethylenetriamine pentaacetic acid-bis-methylamide (DyDTPA-BMA) (sprodiamide injection) were administered intravenously into the left jugular vein as a bolus. Hemodynamic parameters (heart rate, left ventricular pressures, rate of rise of left ventricular pressure [+/- dP/dt], and electrocardiogram as well as central and peripheral pressures) were continuously monitored for 15 minutes after each dose. Left ventricular developed pressure and rate pressure product, as indicators of myocardial oxygen consumption, were calculated. Osmolalities of the injected solutions were determined from freezing-point depression and correlated with the observed hemodynamic alterations. RESULTS Bolus administration of 0.1, 0.3, and 0.5 mmol/kg DyDTPA-BMA produced no significant effect on the various hemodynamic parameters. (NMG)2DyDTPA caused dose-dependent attenuations in heart rate, left ventricular pressures, +/- dP/dt, rate pressure product and arterial blood pressures in both normal and infarcted rats. The magnitude of the response was dose dependent. Significant correlations were observed between osmolality and peak change of hemodynamic variables (r values between 0.99-1.00) after the administration of (NMG)2DyDTPA, but not after the injection of DyDTPA-BMA. CONCLUSIONS Bolus administration of (NMG)2DyDTPA resulted in transient negative inotropic and chronotropic effects and hypotension in both healthy and infarcted animals. DyDTPA-BMA, administered as a bolus even at high doses, caused no appreciable hemodynamic alterations.