The findings presented in this article pertain strictly to the specific conditions of the studies and should be extrapolated to human patients with caution for several reasons. First, the experimental model of a single acute coronary stenosis is a rare clinical occurrence, and it ignores the influence of the well-developed collateral circulation and impaired endothelial function which may accompany long-standing coronary artery disease (22). Second, the absolute hematocrit at which cardiac dysfunction arises would likely vary if a different anesthetic technique or diluent were used, or if the study were carried out in the absence of thoracotomy and positive-pressure ventilation or at a reduced body temperature. Nevertheless, the present findings provide broad guidelines for assessing when hemodilution may be used safely as a method of blood conservation. They suggest (1) that relatively severe hemodilution alone or moderate hemodilution combined with controlled hypotension with either SNP or ADEN may be used safely if cardiac function is normal and the coronary circulation is not obstructed, but (2) that even moderate hemodilution alone may be unsafe in patients in whom the coronary vasodilator reserve is exhausted or severely depleted by a proximal stenosis.