During breathing at rest in humans, electromyographic activity of the expiratory muscles (EMGem), tidal volume (VT), and minute ventilation (V(I)) are higher when standing than when supine. EMGem is known to correlate with VT and V(I). It is not known whether increased EMGem when standing results directly from the change in posture or indirectly from postural changes in ventilatory pattern. Moving average electromyographic activity of the transversus abdominis (EMGta) was recorded using a pair of fine wire electrodes during carbon dioxide (CO2) rebreathing both while standing and while supine. At matched end-tidal CO2 (ETCO2), VT, or V(I) values, EMGta was significantly higher when standing than when supine. Postural differences in EMGta had no correlation with increasing ETCO2, VT, or V(I) during CO2 rebreathing. These results suggested that both the direct effect of the postural change and an indirect effect through changes in ventilatory pattern contribute to the increased EMGem observed when standing compared to that when supine.