The effects of calcium deficient diet and acetazolamide on the gas exchange characteristics of avian eggshells were independently investigated in two groups of unmated hens (Gallus domesticus). In one group, eggs were collected during both a normal diet (3.00% Ca) and a calcium deficient diet (0.34% Ca). In another group, eggs were collected both before and after acetazolamide administration (200 mg/kg) per os. Eggshell water vapor conductance (GH2O) increased 30% during the calcium deficient diet and was accompanied by a 21% decrease in eggshell thickness (L). Eggshell GH2O increased 200% one day after acetazolamide administration and was not only accompanied by a 36% decrease in L, but also by an 89% increase in total functional pore area (Ap). We conclude that a calcium deficient diet increases GH2O by eggshell thinning with little effect on Ap. On the other hand, acetazolamide profoundly increases GH2O, not only by eggshell thinning but also by a remarkable increase in Ap.