Captive mallards (Anas platyrhynchos), fed an all-grain diet for up to 5 months during the winters of 1991 to 1992 and 1992 to 1993, developed lesions of squamous metaplasia; some had no detectable hepatic vitamin A. Vitamin A deficiency in mallards was defined as hepatic levels of retinyl palmitate < 2 micrograms/g liver. Lesions were found only in ducks with low levels of hepatic vitamin A, but not all ducks with these low levels of hepatic vitamin A had histological lesions. The prevalence of lesions in the esophagus was greatest cranially and caudally and less common in the central region. Palatine salivary glands rarely were affected. Mallards with liver stores > 600 micrograms of hepatic retinyl palmitate per g liver, fed a diet deficient in vitamin A were unlikely to become deficient over a 5 month period. Birds fed an all-grain diet had significantly lower vitamin A concentrations in their liver compared to those fed an all-grain diet with vitamin A added. Liver weight, when corrected for body size, did not affect vitamin A concentration. Serum retinol levels were conserved over a large range of hepatic vitamin A levels but levels below 300 micrograms retinol/l were useful in detecting vitamin A deficiency in captive mallards. Based on the findings, the presence of lesions provides a conservative measure of vitamin A status in ducks and tissue levels should be measured in instances when mallards have questionable vitamin A status.