The isolated working rat heart is a useful experimental model which allows contractile function to be measured in hearts perfused at physiologically relevant workloads (refer to Chapter 2, Volume 1). To maintain these high workloads the heart is required to generate a tremendous amount of energy. In vivo this energy is derived primarily from the oxidation of fatty acids. In many experimental situations it is desirable to perfuse the isolated working heart in the presence of physiologically relevant concentrations of fatty acids. This is particularly important when studying energy metabolism in the heart or in determining how fatty acids alter the outcome of myocardial ischemic injury. 1 , 2 However, perfusing hearts with fatty acids is complicated by the fact that the lipophilic nature of most fatty acids does not allow the investigator to easily dissolve the fatty acids in the perfusion buffer. The fatty acids predominately oxidized by the heart (palmitate and oleate) are extremely water insoluble. Nature overcomes this obstacle in vivo by transporting fatty acids in the blood either bound to albumin or as triacylglycerols contained in lipoproteins. In the isolated working rat heart, both of these carrier systems have been used, 2 although fatty acids bound to albumin is the most common experimental system used for delivering fatty acids to the heart in isolated heart experiments. The use of perfusion buffers containing fatty acids bound to albumin remains a challenge, since albumin-containing solutions cannot be directly oxygenated using classical methodology (see Chapter 2, Volume 1) because direct gassing of the buffer will create excess foaming. However, using appropriate perfusion apparatus and perfusion techniques, this problem can be easily overcome.