G protection afforded by the abdominal bladder of a pneumatic anti-G suit is usually attributed to counter action of +Gz-induced 1) caudad displacement of the heart and 2) pooling of blood in abdominal veins. The present study examined whether the abdominal bladder might provide G protection also via other mechanisms. Subjects were exposed to +Gz loads while sitting relaxed and wearing a full coverage anti-G suit modified to permit separate pressurization of the abdominal and leg bladders. In two experimental series (n=8, n=14) subjects were breathing at positive airway pressure (PPB). In a third series (n=5) subjects were breathing at atmospheric airway pressure. Pressure in the lower thorax was estimated by use of an esophageal catheter. Mean arterial pressure (MAP) was measured using a volume-clamp technique. During PPB at high G-loads intrathoracic pressure was higher with than without pressurized abdominal bladder. In 7 of the 14 subjects, intrathoracic pressure exceeded airway pressure during PPB when the abdominal bladder was pressurized. The MAP response at high Gz loads was higher in this subset of subjects than in the subjects in which airway pressure exceeded intrathoracic pressure. Without PPB at increased Gz load, intrathoracic pressure was higher with than without pressurized abdominal bladder. Thus, during PPB the abdominal bladder acts as an airway counter pressure, thereby facilitating transmission of pressure from the airways to the thorax and hence improving G protection. It also appears that in several individuals, pressure may be transmitted from the abdominal bladder into the lower thorax and to the heart.