Abstract Eggshell thinning in ducks was induced by administration of p, p′-DDE in the diet (40 mg/kg food) for 45 days. This treatment resulted in a 19% reduction of the Eggshell Index (EI). Shells from calcifying eggs obtained at the time of slaughter showed a 36% reduction of EI. Prostaglandin synthesis by a homogenate of eggshell gland mucosa from p, p′-DDE-treated ducks was reduced by 24%. HCO 3 − -stimulated ATPase activity by a homogenate of eggshell gland mucosa from p, p′-DDE-treated ducks was not significantly changed. The calcium content of eggshell gland mucosa was increased to 127% in p, p′-DDE-treated ducks. p, p′-DDE-treated ducks showed profound changes in the shell gland luminal content of several ions. Calcium (−43%), sodium (−15%), potassium (−15%), bicarbonate (−33%) and chloride (−29%) were all significantly reduced in p, p′-DDE-treated ducks. The content of phosphate was unchanged. These findings are discussed in relation to a proposed mechanism for p, p′-DDE-induced eggshell thinning that involves inhibition of prostaglandin synthesis in eggshell gland mucosa.