The effects of a 1- or 24-hour pretreatment regimen with monophosphoryl lipid A (MLA, 35 micrograms/kg i.v.) on myocardial stunning produced by repetitive coronary occlusions were studied in barbital-anesthetized dogs. Regional segment function (%SS) and myocardial blood flow were measured by sonomicrometry and the radioactive microsphere technique, respectively. In controls, six 5-min periods of coronary occlusion interpersed with 10-min periods of reperfusion and ultimately followed by 2 h of reperfusion produced regional segment dysfunction. Pretreatment with MLA for 1 h prior to the first occlusion period had no effect on %SS, however, pretreatment with MLA 24 h prior to the first occlusion period resulted in a significant (p < 0.05) improvement in the recovery of %SS at all reperfusion periods as compared to the control group. In addition, segment dysfunction during each occlusion period was significantly less severe in animals receiving a 24-hour pretreatment with MLA as compared to the controls. These results are the first to demonstrate that MLA, a lipid A derivative of endotoxin, preserves contractile function of ischemic myocardium in an in vivo canine model and that its cardio-protection is time dependent.