Pores of the avian eggshell provide the only communicating channels for the exchange of molecules between the developing embryo and the external environment. These trumpet-shaped ducts are microscopic. Their number and effective pore radius increase with egg mass from ca. 300 and 3 microns in 1 gm eggs to about 30,000 and 13 microns, respectively, in 500 gm eggs (Ar and Rahn, Respir. Physiol. 61:1-20, 1985). The total volume of all the gases that diffuse through these pores is large. For example, up to the time when internal pipping takes place, when pulmonary ventilation is initiated, about 20 liters (O2 + CO2 + water vapor) have passed through 10,000 pores of an 80 gm egg. The rules governing this exchange of gases are best described by Fick's first law rather than by Stefan's law, which describes the transfer of gases across stomata of plants. Data are presented for those species for which the water vapor conductance, the oxygen flux at the preinternal pipping stage, the rate of incubation water loss, and the number of pores in the egg have been established. These conductances and flux rates are directly proportional to the number of pores. These relationships suggest that, in spite of differences in egg mass and incubation time, in most birds the average conductance of individual pores is similar, namely, 1.5 microgram (d X torr)-1 and that the average O2 and water vapor flux per pore are 60 and 54 microliters X d-1, respectively. These values are similar to the estimates previously presented (Ar and Rahn, '85).