Abstract: Melatonin (0.3 to 0.4 mg/kg) dissolved in 0.5 ml dimethylsulphoxide (DMSO) was injected i.v. into six baboons, and their cardiovascular parameters were monitored. Left ventricular end-diastolic and end-systolic volumes, stroke volume, cardiac output, and left ventricular ejection fraction were measured, using conventional radionuclide ventriculography, and compared to normal values previously established. These parameters were also measured after an i.v. administration of only DMSO. The only statistical significant change due to melatonin was the increase in the cardiac output and left ventricular ejection fraction. With the reduced heart rate the increase in cardiac output implies a positive inotropic action on the heart by melatonin. There are indications that DMSO possibly suppresses cardiovascular actions of melatonin.