Constant-pressure manometry, previously designed to study O2 and CO2 gas exchange in small pieces of tissue, cells, and organelles, was adapted to study bulky organs. According to this new procedure, a near-zero-volume Devaux chamber connects a manometer to the internal atmosphere volume (VG) of a plant organ covered by a layer of epoxy, submerged in unstirred water, kept at constant temperature, and kept at the same VG pressure. Equations, based on CO2 and O2 solubility at equilibrium with VG, were used to follow O2 consumption as a function of reduced internal O2 pressure over time [for organs with VG < 0.1 (v/v) and respiratory quotient (RQ) of 0.7 to 1.3] to observe the transition between aerobiosis and anaerobiosis and to measure CO2 evolution during the anaerobic phase. For those measurements, bulky-organ manometry performed consistently in tomato [VG = 6.41% (v/v)], sweetpotato [VG = 8.57% (v/v)], and potato [VG = 0.34% (v/v)]. The results indicate that constant-volume manometry is sufficiently precise to detect differences in respiratory metabolism as a function of intercellular O2 concentration in intact plant organs.