The contractile function of rat pulmonary arterial vessels, ranging in internal diameter from 100 to 2000 microns, was studied in vitro using a small vessel myograph. It was found that vessels with a diameter of 200-400 microns produced considerably more force for a given intervention than those either smaller (less than 100-2000 microns) or larger (400-2000 microns). It is suggested that these vessels may play an important role in the generation of pulmonary vascular resistance.