Normalization of gas exchange and growth of chicken embryos incubated under abnormal conditions with regard to eggshell conductance and barometric pressure is directed to restoring the normal water loss from the eggs and the normal pattern of air space gas tensions during development. The latter is achieved by maintaining an O2 tension of 149 torr in the gas mixture supplied to the incubator and by reducing the incubator ventilation rate to compensate for increased, standard or effective, eggshell conductances. Thus, for maintenance of normal air space gas tensions, a decrease of the diffusive resistance of an egg requires a similar increase of the convective resistance so that the total resistance against gas transport remains normal. The latter must be set at a value given by (149-PA)/M, where PA is the normal air space O2 tension, torr, and M is the normal O2 uptake, ml(STPD)/day. This value is independent of embryonic age, because PA is directly proportional to M during development. Thus the convective resistance required for maintenance of normal PA and M values is given by the difference between (149-PA)/M and the diffusive resistance. Such PA and M values can be maintained in eggs with the minimum diffusive conductance, M/(149-PA), thus requiring an infinite convective conductance, to infinite diffusive conductance, which requires a minimum convective conductance, M/(149-PA). This minimum is predicted for all shell-less eggs, viz., for the continuous replacement of diffusive by convective gas transport.