The Système International d'Unités (SI) is progressively being adopted in all branches of science. Physiology and biophysics are no exceptions. Resistance to the use of SI-units derives from the ingrained habit of utilizing traditional units, which may appear simpler but have many limitations. The SI not only simplifies units but also involves the standardization of spelling, symbols and presentation. This article describes the relationship between the SI and the units commonly used in cardiovascular and pulmonary physiology and biophysics. It provides conversion factors for SI units in the hope that it will stimulate a wider use of these new international units.