The clearance of nitrogen-13 (13N) from the upper, mid and lower zones was measured with a gamma camera during spontaneous breathing in 10 seated subjects. The clearance was monitored after (a) an intravenous injection of 13N dissolved in saline and (b) equilibration with 13N gas in closed circuit. Subjects breathed air first, and then a 30 or 11% oxygen mixture. For any region, the time for 90% elimination (T90) was related to the volume expired by the whole lung during that time (VE90). For the mid and lower zones, the clearance was faster (VE90 smaller) after intravenous 13N than after equilibration with 13N gas. This difference persisted when 30% or 11% oxygen was inspired. For the lung overall, the physiological dead space for a mean tidal volume of 884 ml was 277 ml for intravenous 13N clearance, and 384 ml for 13N clearance after equilibration. The conclusions drawn for this study are (1) ventilation in relation to volume is uneven within lung regions (2) intraregional perfusion in relation to volume is also uneven (3) at a local level the well-ventilated units are better perfused (4) these inhomogeneities are not affected by raising or lowering the inspired oxygen concentration.