Left and right ventricular end diastolic pressure, stroke work, stroke volume, and cardiac output were measured in open and closed chest dogs infused with blood and/or saline. The methods employed permitted repeated determinations of these factors during short time intervals. For nonfatigued hearts in the open chest preparation point to point correlation of end diastolic pressure with the other three variables was very good, but in closed chest animals a number of non-correlations occurred. Sufficient variation was seen to indicate that factors other than those measured in this study will have to be known before cardiac performance in the intact animal can be fully assessed.