Abstract 1. A recent report from the present laboratory indicated that the DDT-metabolite p, p'-DDE, which produces eggshell thinning in sensitive species of birds, inhibits prostaglandin synthesis in the eggshell gland. 2. This was found to be true both after in vitro addition to a homogenate of eggshell gland mucosa from ducks, and after in vivo administration to ducks at a dose regimen that produces eggshell thinning. 3. The purpose of the present study was to compare the p, p'-DDE-sensitive duck and the insensitive domestic fowl regarding the effects of p, p'-DDE administration on calcium and prostaglandin metabolism of the eggshell gland mucosa. 4. Administration of p, p'-DDE caused significant eggshell thinning in ducks (−18.5%) but not in the domestic fowl. 5. The shell thinning in ducks was accompanied by significant reductions in 45Ca uptake and prostaglandin synthesis by a homogenate of the eggshell gland mucosa, as well as significant reductions of the prostaglandin E 2 content in the shell gland and the content of calcium and bicarbonate present in the shell gland lumen at the time of slaughter; the content of calcium in the eggshell gland mucosa was significantly increased. 6. None of the changes was observed in the domestic fowl. 7. Based on these findings, inhibition of a prostaglandin-stimulated bicarbonate transport across the eggshell gland mucosa is discussed as a possible mechanism for p, p '-DDE-induced eggshell thinning in birds.