The mechanism by which the avian shell gland (SG) transports large amounts of Ca (and HCO3) for assembly of an eggshell was explored. Experiments were conducted in SGs perfused in situ; unless otherwise stated, hens were secreting shell (active) when selected for use. Compared with another part of the oviduct, the magnum, the SG extracellular compartment is greater and Ca is distributed in a larger total volume of which most is extracellular. Intracellular distribution of Ca is similar in the two organs. Water content and vascular space/g is always larger in the SG and, when an eggshell is forming, SG vascular space increases 40%. Radioactive Ca in the circulation rapidly appears in the SG lumen at rates closely reflecting Ca clearance in the blood. Circulating inulin-1 4C also appears in the lumen in significant quantities, although at slower rates than Ca. Inulin penetration rate to the lumen was 4-fold higher in active than in inactive SGs. Collectively, these observations are consistent with the presence in the SG of highly permeable, paracellular shunt pathways for passive fluxes and that these paths become more permeable during shell formation. Perfusion of the SG lumen with physiologically balanced ionic solution showed that Ca, HCO3 and K normally are secreted, while Na and Cl are absorbed. As the perfused Ca concentration was varied artificially, Ca secretion was somewhat higher in the absence of perfused Ca, but secretion continued at constant rate against a considerable plasma:lumen concentration gradient of Ca. Also, increasing the Ca concentration tended to increase HCO3 secretion. Increasing perfused HCO3 raised Ca secretion while sharply lowering HCO3 secretion such that at high concentration, net absorption of HCO3 occurred. Acetazolamide reversibly abolished Ca and HCO3 secretions while only slightly affecting Na and Cl absorption. Raising perfused Na increased Ca secretion and sharply increased Na absorption. Ouabain changed Na and Cl absorption to secretion, while only depressing Ca and HCO3 secretion 60%. Effects of ouabin were completely reversible. Net water flux to the lumen, induced by increasing luminal osmotic pressure with a nonelectrolyte added to the usual salts, depressed Na and Cl absorptions while slightly increasing HCO3 secretion. Where measured in the ion experiments, the transmural electrical potential difference was small (10–16 mv), did not change significantly and favored cation transport to the lumen. Potassium secretion was not affected by any manipulation of the luminal environment. These results suggest that Ca secretion occurs both by diffusion and, more importantly, by active transport. Net Ca secretion does not appear to be critically dependent upon Na absorption. However, Ca secretion does appear to be linked functionally to luminal HCO3 concentration and upon HCO3 production. HCO3 secretion depends more upon a concentration gradient than does Ca secretion, but HCO3, too, is actively transported.