Right ventricular function has been studied in a canine ejecting heart model in-situ with intact sympathetic innervation and constant cardiac output, during varying degrees of left heart bypass ratio ranging from nil to total left ventricular decompression. The right ventricular function deteriorates as the left heart bypass ratio increases, reaching the maximal degree of RV depression when the LV is maximally decompressed. Some practical implications for the conduction of left heart bypass are discussed.