A biaxial bubble test has been designed to ascertain the mechanical properties of rat pulmonary arteries. The analytical procedure used to estimate stress and strain from the resulting test data is presented along with some analytical results. The bubble test was performed by loading a flat piece of rat pulmonary artery into a test fixture beneath a circular opening; the material was subsequently pressurized from below, producing a "bubble" of deformed material. Due to the anisotropy of the rat pulmonary artery, the resulting bubble was ellipsoidal in shape. Test results were recorded in the form of side-view images taken from various angles at incremental values of pressure. Average strains were estimated with the use of image analysis to measure changes in the bubble perimeter during inflation. Formulations for isotropic materials were applied to estimate stresses based on the anisotropic geometry of the bubbles produced during testing; some results of this preliminary analysis are presented here. Results from this analysis show differences in mechanical properties of the rat pulmonary artery from those of healthy versus hypertensive rats.