The article describes a method for monitoring the total energy output and oxygen uptake of isolated perfused rat hearts with working left ventricles. Twenty-two unpaced hearts (rates 4-4.5 bs-1) were separately investigated inside a flow micro-calorimeter (one minute for 90% thermal response) at 37 degrees C. They pumped fluid into an artificial arterial system with adjustable linear peripheral resistance and variable volume compliance. After about 20-40 minutes a steady state period was achieved and most of the hearts continued to operate in this state for a further 40-100 minutes. In the steady state the outputs were in the ranges of 30-50 mJ s-1g-1 mechanical power (per gram tissue dry weight) and 100-300 mJ s-1g-1 heat production at an oxygen uptake of between 0.01 and 0.02 cm3 O2 s-1g-1. This resulted in approximately constant cardiac outputs between 2 and 4 cm3 s-1g-1 aortic and coronary fluid and stable mechanical efficiencies between 12 and 20%. The energy balance in steady state under a number of defined arterial loads was also analyzed. The hearts attained reproducible maxima of mechanical efficiency at specific loads. Methods to allocate reference points in the energy scheme are discussed. The yield of biochemical energy from the perfusion fluid (utilized for contraction and heat production) was (on average) 21 J per cm3 oxygen consumption (energy equivalent of oxygen). No obvious correlation between this value and the mechanical efficiency was evident.