Because conflicting results have been obtained in studies of the hamster (Mesocricetus auratus) microcirculation following rapid decompression, we were interested in the origin of bubbles in the cheek-pouch preparation. The hamster cheek-pouch and surgically exposed femoral vessels were observed under the microscope, before and immediately following rapid decompression. The animals were placed in a hyperbaric chamber and exposed to an increased pressure of 7 ATA for 1 h. They were then decompressed at a rate of 18 m/min until reaching 1 ATA. This hyperbaric exposure produced an LD50 in 100 hamsters examined in this study. In 40% of these animals "bubbles" were seen entering the cheek-pouch artery after a delay of from 3 to 6 min following decompression. Before observing bubbles in the cheek-pouch artery, bubbles were always seen in the femoral vein. Those animals that had observable bubbles in the cheek-pouch usually demonstrated a tachypnea, followed by gasping and apnea. When bubbles were seen in the arterial circulation, few animals survived for more than 10 min following the hyperbaric exposure. Postmortem examination of these animals revealed massive gas emboli in the large venous vessels, right ventricle, and fewer bubbles in the left ventricle. Postmortem examination of surviving animals revealed that the majority of the gas was in the venous vessels and right heart. These data support the hypothesis that bubbles first form on the venous side of the circulation and, if they exceed a certain volume, move through the pulmonary circulation into the systemic arterial vessels.