The Penn State Intravascular Lung (PENSIL) consists of thousands of microporous, hollow, polypropylene fibers, each with one end sealed and the other end attached to an oxygen (O-2) supply tube which is implanted in a major vein. Since diffusion resistance along the inside of the blind-ended fibers is significant, a hypobaric O-2 supply might enhance the desorption of carbon dioxide (CO2) from blood. Therefore, CO2 desorption through small-scale PENSIL modules containing 14 polypropylene fibers, 380 mu m in diameter and from 0.5 to 5.0 cm in length, was measured as the O-2 supply pressure was varied from 300 to 900 Torr. The fiber modules-were mounted in a 1.75-in.-diameter transfer chamber in which water saturated with 760 Torr of CO2 flowed around the fibers at 1.2 or 3.4 L/min. At atmospheric pressure,the CO2 desorption rate leveled off with increasing fiber length. At hypobaric conditions, however, the rate of CO2 desorption increased linearly with fiber length, and was as much as eight times greater than at atmospheric pressure. These data were analyzed with a one-dimensional binary diffusion model that utilized an overall mass transfer coefficient as a free parameter. The best estimates of the coefficient (20-40 mu m/s) were independent of gas pressure but varied with fiber length.