Are both gas exchange and gas tensions uniform in different regions of the developing hen's egg? To answer this question we measured the O 2 uptake and CO 2 production of the whole egg, and at the same time the O 2 and CO 2 tensions of the air cell. The gas exchange ratio (R) of the whole egg differed from R calculated from air cell P O 2 and P CO 2 values, in agreement with the findings of Visschedijk [Br. Poultry Sci. 9: 173–184 (1968)], who measured gas exchange separately over both the air cell region and the remainder of the egg. We constructed a diffusive shell conductance/perfusion (G/Q̇) line on the O 2-CO 2 diagram from a blood nomogram for the chick embryo in late development [Olszowka et al., Fed. Proc. 46: 512 (1987)], and used this to analyze our results. The G/Q̇ ratio for the area of shell over the air cell differs from that for the remainder of the egg. Our analysis permits us to calculate, for each area, the regional shell conductance, blood flow, and O 2 and CO 2 tensions in the gas spaces between the shell and the chorioallantoic capillaries.