Although gas exchange across the avian eggshell has been treated as a simple diffusion process heretofore, the nonequimolar nature of diffusive fluxes engenders a convective flow and thus causes an overpressure (delta Ph) within the shell relative to the surrounding atmosphere. The convective flow must be taken into account in assessing the driving forces and corresponding flows of the gases involved. Using the nonmetabolizing hen's egg as a model, we describe the interactions of diffusion and convection as they affect water vapor and inert gas exchange through the pores. A typical infertile hen's egg containing water vapor at 50 torr and immersed in dry air (the situation in a desiccator at 38 degrees C) will lose about 900 cm3 d-1 of water vapor by a combination of diffusion and convection. Diffusion is the predominant process, accounting by calculation for 96% of total water loss as vapor. The remaining 4% of water vapor exits by convection, which also carriers with it 480 cm3 d-1 of air. We measured delta Ph caused by this total convective flow across the shell in air, He-O2, and He, and found values of about 1, 4, and 8 mm H2O, respectively. A theoretical treatment of gas flow through pores based on Fick's and Poiseuille's laws yields delta Ph values somewhat lower than those measured. Possible reasons for the discrepancy are discussed, together with implications of our findings for gas exchange and conductance measurements in metabolizing eggs.